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I know modern Macs all have USB3 support, but now there's USB 3.1, USB 3.2, NVMe and of course thunderbolt (presumably multiple versions) and I am lost.

What does the 2020 Intel iMac 27" support? What does that mean for data transfer speeds? When buying an external drive what do I need to look out for to make sure it will be compatible with the fastest option?

4
  • What kind of research have you already done on this (e.g. "What does the 2020 Intel iMac 27" support" seems to be answerable from looking at the specs) , what are the options you have?
    – nohillside
    Jul 23 at 10:01
  • I have read the spec sheet and it left me confused. It mentions thunderbolt, thunderbolt2 and thunderbolt3, and "USB3.1 gen 2". I don't really understand what THunderbolt is in the first place.
    – Mr. Boy
    Jul 23 at 10:13
  • I’m interested in your use case. The “fastest” will be extremely expensive, so unless you’re sure you need it…?
    – Tim
    Jul 24 at 11:28
  • @Tim we might say "fastest reasonable" but really I'm after a comparison the different options. David and Gilby cover this nicely; Peter takes the "absolute fastest" angle and does a great job explaining how to get the maximum.
    – Mr. Boy
    Jul 24 at 11:39
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There are three things which effect compatibility and performance when connecting external storage:

  1. The physical connector and cable. The 2020 iMac has four USB Type A (USB-A) and two USB Type C (USB-C) connectors.

  2. The data transfer protocol - USB 1, USB 2, USB 3 (and later variants 3.1 and 3.2) and Thunderbolt. There is also USB 4, but this is not relevant to 2020 iMac. The 202O iMac supports:

  • USB-A ports: USB 3.0 (maximum speed 5 Gbits/s) and slower USB 2 and USB 1.
  • USB-C ports: Thunderbolt 3 (maximum speed 40 Gbits/s), USB 3.1 gen2 (10 Gbits/s), and slower USB 3.1 speeds.
  1. The drive inside the external device/enclosure. Common types are are SATA (maximum speed 6 Gbits/s) or NVMe (faster). HDD are usually SATA, whilst SSDs might be SATA or NVMe.

The 5 Gbit/s USB 3.1 gen1 and 3.2 gen1x1 standards are similar to USB 3.0 and storage devices specifying these are generally compatible with USB 3.0 via the USB-A ports.

Further, the 10 Gbit/s USB 3.2 gen2x1 is very close to USB 3.1 gen2. So storage devices specifying USB 3.2 gen2x1 should be compatible with the 10 Gbit/s USB 3.1 gen2 protocol delivered by the USB-C ports.

As well as the the iMac ports supporting or being compatible with multiple data protocols, most USB storage devices support multiple data protocols and (in many cases) both USB-A and USB-C cables and computer ports.

For an overview of USB standards read Wikipedia.

Thunderbolt is a different connection standard developed by Intel (in collaboration with Apple) when Apple saw a need for a faster connection than that provided by USB 3. The first two iterations of the standard used the Mini DisplayPort physical connector on iMacs, whereas Thunderbolt 3 uses USB-C.

Thunderbolt and USB standards have now converged with Thunderbolt 4 and USB 4 being very similar. For more detail, read Wikipedia. The current 2021 iMacs support Thunderbolt 3 and USB 4 (using USB-C physical ports).

For the 2020 iMac, Thunderbolt 3 storage devices can only be connected to the USB-C ports and must use Thunderbolt 3 cables. Older Thunderbolt 1 and 2 devices can only be connected using cables which convert to Thunderbolt 3 - the iMac does not directly support Thunderbolt 1 and 2.

Note that physical USB-C ports which support Thunderbolt 3 have a thunderbolt like symbol next to them.

To further confuse matters (but out of scope for this question) USB-C ports also support connecting monitors using, for examples, Display Port and HDMI standards.

What should you be looking for:

For maximum speed (and cost) you should be looking for a Thunderbolt 3 SSD storage device. These can have performance similar to the internal SSD (~2000 MByte/s). Example: Samsung X5.

For a more modest cost, there are SSDs which use USB 3.1 Gen 2 (or 3.2 Gen 2x1) and via the USB-C ports on the iMac will deliver ~1000 Mbyte/s. Example: Sumsung T7.

And for a slightly more modest cost there are SSDs which use USB 3.1 Gen 2, but internally use SATA SSDs and deliver only ~500 Mbyte/s. Example: Samsung T5. These devices can connect to either USB-C or USB-A ports using USB 3.0, 3.1 Gen1/2 or 3.2 Gen1/2 - the differences in speed are minor as the SATA SSD becomes the bottleneck.

Unless you have a professional need for highest speed, devices like the Samsung T7 (or T5) provide fast secondary storage.

I have just used Samsung as examples. There are other equally good (and some not quite so good) brands.

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  • Sequential bandwidth isn't the only criterion for storage speed. NVMe has some inherent protocol / driver advantages for low latency and high number of small IO operations per second. So I'd expect an enterprise-grade PCIe SSD connected over Thunderbolt 3 to be king, if you can connect Thunderbolt to an external-GPU enclosure but use the slot for storage instead. Possibly even an Intel Optane DC using 3D Xpoint storage, like intel.ca/content/www/ca/en/products/sku/201859/… (2.5" NVMe PCIE4.0 x4) Jul 24 at 5:40
  • 1
    Giliby: You refer to USB 3.2 Gen 1 and USB 3.2 Gen 2. In terms of speed, I can not find these in your Wikipedia reference. I can find USB 3.2 Gen 1x1, USB 3.2 Gen 2x1, USB 3.2 Gen 1x2 and USB 3.2 Gen 2x2. In this a mistake on your part or am I missing something? Jul 24 at 6:32
  • @DavidAnderson You are, of course, correct. I was deliberately ignoring the x1 and x2 distinction as the x2 is not supported by the 2020 iMac. I will make changes.
    – Gilby
    Jul 24 at 9:10
  • @DavidAnderson I have also slightly changed/expanded the Thunderbolt section because the OP was confused as to what it is. Please check the changes.
    – Gilby
    Jul 24 at 9:56
2

I have a 2018 Mac mini which has the same USB and Thunderbolt 3 capabilities as your Mac. I also have a Thunderbolt 3 Samsung X5 SSD and a 10 Gb/s USB Samsung T7 SSD. I will base my answers on what I know about this hardware.


When the Thunderbolt 3 Samsung X5 is connected to the Mac through a USB Type-C receptacle (jack), System Information displays the following under NVMExpress.

NVMExpress

Like the internal drive, the Thunderbolt 3 Samsung X5 appears as a NVMExpress drive, except the drive is external. Note that TRIM is supported.

System Information also displays the following under Thunderbolt.

Thunderbolt 3

The Thunderbolt 3 Samsung X5 has a read speed of 2800 MB/s. In terms of bits per second, this would be 22.4 Gb/s, since 8 bits = 8 byte. Since the speed shown in the image above is up to 40 Gb/s, one can conclude the read speed of 22.4 Gb/s is not limited by Thunderbolt 3, but rather by the NVMe drive inside the Samsung X5.


When the USB Samsung T7 is connected to the Mac through a USB Type-C receptacle, System Information displays the following under USB.

Type-C

The data transfer speed is unto 10 Gb/s. This is the maximum speed for USB devices connected a USB Type-C receptacle on both of our Macs. There is no indication of TRIM support, which may not be necessary if the embedded PCIe NVMe technology is sufficiently fast.


When the USB Samsung T7 is plugged in a USB Type-A receptacle on the Mac, System Information displays the following under USB.

Note: The USB Samsung T7 comes a cable for plugging into a USB Type-C receptacle and a different cable for plugging into a USB Type-A receptacle.

Type-A

The data transfer speed is unto 5 Gb/s. This is the maximum speed for USB devices connected a USB Type-A receptacle on both of our Macs. Again, there is no indication of TRIM support.


Conclusions

You post contains the following four questions.

  • What is the fastest external storage supported on Intel 2020 27" iMac?

    The Thunderbolt 3 Samsung X5 has a read speed of 2800 MB/s, which is 22.4 Gb/s. This means the data rate between the external drive and the Mac must exceed 22.4 Gb/s, which is faster than the maximum USB data rate of 10 Gb/s offered my your Mac. From this, the following conclusion can be made.

    >Nothing that USB offers will be faster than what Thunderbolt 3 offers and also be compatible with your Mac.

  • What does the 2020 Intel iMac 27" support?

    The The USB Type-C receptacles support at least the following devices.

    Self powered Thunderbolt 1
    Self powered Thunderbolt 2
    Thunderbolt 3
    USB 2.0
    USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)
    USB 3.1 Gen 2 (same as USB 3.2 Gen 2×1)

    Note: Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter does not provide power from the Mac to the device.

    The The USB Type-A receptacles support at least the following devices.

    USB 1.1
    USB 2.0
    USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)

  • What does that mean for data transfer speeds?

    The USB Type-C receptacles offer Thunderbolt 3 speeds up 40 Gb/s and USB speeds up to 10 Gb/s.

    The USB Type-A receptacles offer USB speeds up to 5 Gb/s.

  • When buying an external drive what do I need to look out for to make sure it will be compatible with the fastest option?

    Look for a Thunderbolt 3 SSD with high with read/write speeds (in the GB/s range). The SSD should also support TRIM. You can also look for a Thunderbolt 1 and 2 SSD, but these drives would need to be self powered and would require a Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. However, finding a self power Thunderbolt 1 or Thunderbolt 2 SDD at a price lower than Thunderbolt 3 SSD is unlikely.


Speed Comparisons

T5
External SSD
T7
External SSD
X5
External SSD
27" 2020 iMac
Internal SSD
27" 2020 iMac
Internal SSD
Source The Verge StorageReview The Verge Tom's Guide PC Magazine
Capacity 1 TB 2 TB 1 TB 1 TB 1 TB
Benchmark
Applications
Novabench
and
Blackmagic
Blackmagic Novabench
and
Blackmagic
Blackmagic Blackmagic
Average
Read
518 MBps 894 MBps 2410 MBps 2467 MBps 2427 MBps
Average
Write
475 MBps 840 MBps 1708 MBps 2757 MBps 2735 MBps

Advertised Specifications

T5 External SSD T7 External SSD X5 External SSD
Source Samsung Samsung Samsung
Interface Up to 10 Gbps
(USB 3.2 Gen 2x1)
(USB 3.1 Gen 2)
Up to 10 Gbps
(USB 3.2 Gen 2x1)
(USB 3.1 Gen 2)
Up to 40 Gbps
(Thunderbolt 3)
Read Up to
540 MBps
Up to
1050 MBps
Up to
2800 MBps
Write Up to
540 MBps
Up to
1000 MBps
Up to
2300 MBps
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  • I would love to know how the internal drive compares, if you wouldn't mind adding it alongside the other cases as a 'control'. Not sure if it would similar or much faster than the X5 or what sort of bus it uses internally
    – Mr. Boy
    Jul 25 at 22:11
  • Mr. Boy: I added some tables with respect to speed. I wonder why the read speeds for the external SSDs are faster than the write speeds, but the opposite was measured for the internal drive. Open the System Information application on your iMac to get the type of internal drive. (I assume this is what you meant by stating: "what sort of bus it uses internally ". Jul 26 at 2:01
  • Thanks... the issue is I haven't bought the iMac yet. I am researching this first :) But basically Thunderbolt3 is (give or take) as fast as the internal SSD which means it could run off a primary external drive easily.
    – Mr. Boy
    Jul 26 at 8:24
  • With my 2018 Mac mini, I have macOS installed on the internal drive and all the user files on the external Thunderbolt 3 drive. You can change an user's home directory from the "Users & Groups" pane of System Preferences. Another possibility would be to create a Fusion Container from two partitions. One partition would come from the faster internal drive and the other from the slower external drive. Jul 26 at 13:06
1

If you see Mactracker : https://apps.apple.com/us/app/mactracker/id430255202?mt=12 your Mac have 2 Thunderbolt 3 port :

Thunderbolt 2 - Thunderbolt 3 (up to 40 Gbps)

So it's the fastest on your Mac. The externals enclosures SSD Thunderbolt 3 have inside a NVME SSD. For exemple you have the Samsung X5.

1

The fastest port on your iMac is the Thunderbolt port. It can support data rates as high as 40Gbit/s, which is 5GB/s.

Now most consumer SSDs are pretty fast but won't exceed that theoretical limit of 5GB/s. The fastet you can usually get is about 3GB/s, which you could do with this enclosure and basically any NVME SSD that fits that criteria mentioned in the product page.


About the confusion of Thunderbolt:

Thunderbolt and Thunderbolt 2 was primarily used on Macs and most Windows Computers "only" had USB. Thunderbolt is more versatile than the older USB 2 and USB 1 standard. It is able to support e.g. display connections through the DisplayPort standard which was integrated in the Thunderbolt specs, which normal USB 1 and 2 did not include. Older USB and Thunderbolt ports did not share the same connector so distinction was easy.

Nowadays the USB-C connector is the same as Thunderbolt 3, even USB 4 and Thunderbolt 4 use this connector. However USB-C is not generally as powerful as Thunderbolt 3. The supported protocols however, may differ. I.e. you could have a USB-C connector which only supports the USB 3.1 protocol while an other one looking identical would support USB 3.2.

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  • thanks for the thorough answer it clears things a lot. The one part I am unclear about is how NVMe fits in. Is it comparable to SATA, or PCI, or...? Is it anything to do with Thunderbolt or is it just a way of knowing an external SSD is fast enough to keep up with Thunderbolt's crazy 40Gbit speed?
    – Mr. Boy
    Jul 23 at 10:45
  • "Nowadays the USB-C connector and the Thunderbolt 3 connector look identical" – This is wrong, or at least misleading. There is no Thunderbolt 3 connector. Thunderbolt 3 (and 4) use the USB-C connector. The statement "the USB-C connector and the Thunderbolt 3 connector look identical" is misleading, since it implies that those are different things. They are not. They don't "look" identical, they are identical. The USB-C connector is the Thunderbolt 3/4 connector. Jul 23 at 20:58
  • You're right. I will clarify that. Thank you
    – X_841
    Jul 24 at 12:59
1

If money is no object, an enterprise-grade PCIe SSD via Thunderbolt for >3.5GB/s sequential read or write, and over 1 million IOPS* (IO ops / sec).

Most likely a recent Intel Optane SSD using 3D XPoint memory, not flash, although there are fast enterprise SSDs from other vendors. Numbers in the title are a rough guesstimate of what you might get from an Optane SSD DC P5800X PCIe4.0 drive bottlenecked by PCIe3.0. Or some flash SSD from another vendor that can push up to the limits of PCIe3.0 x4. Or even a bunch of DRAM in a PCIe card, if they still make those.

You said "fastest", not "fastest that might be worth buying for consumer/personal use", so that's the point of this answer: ways in which SSDs can be fast, and how to spend thousands of dollars on a terabyte or more of SSD space that could keep up with many cores on a big Xeon for many workloads.

That machine has a Thunderbolt 3 port, so that gives you a PCIe3.0 x4 connection to work with, much more bandwidth and lower latency / less overhead than USB3.1 or even 3.2, even with UASP (USB-attached-SCSI that bypasses some of the USB protocol overhead and limitations). With the right adapter, that should let you connect an M.2 SSD (like modern laptops and desktops use for high-performance SSDs). Or with something designed as an external GPU enclosure, should give you a PCIe x16 card slot (electrically only x4 from Thunderbolt) with cooling fan, letting you use enterprise SSDs with serious heat-sinks designed to go in motherboard PCIe slots. e.g. like this one that came up in a search result.

(Sustained SSD performance typically involves some serious heat in the controller, e.g. 21W for a high-end Intel Optane DC like P5800X, so a heat sink is required to allow continuous operation without having to throttle to avoid overheating. M.2 consumer drives often will need to throttle if you run them hard, but usually that only happens during artificial benchmarks; real use is typically bursty like copying a few tens of GB for a few seconds and then going back to idle.)

The NVMe driver interface standard for computers to talk to SSDs means that you wouldn't (AFAIK) need a special driver to use enterprise-grade SSDs with a Mac. (The very fastest non-volatile storage memory is probably Optane DC PM that comes in DIMMs that you plug into memory slots (like this), for use with Cascade Lake and later server CPU, letting user-space processes truly map the storage into their own address space, bypassing the kernel and letting drivers control access via virtual memory page permissions. It's pretty neat, but you couldn't use it with an iMac).

SSDs that use 3D XPoint instead of Flash can be very fast, especially for small writes and for mixed read+write workloads, so that's what you'd want to go for in this hypothetical scenario. SSDs built around 3D XPoint instead of flash are definitely optimized for speed over capacity and price.

PCIe is backwards compatible: a drive capable of PCIe4.0 plugged into a slot / cable / adapter only capable of PCIe3.0 will negotiate the fastest speed both sides are capable of, so a super high-end drive like an Intel Optane SSD P5800X (2.5" NVMe PCIE4.0 x4) (review) can still run at PCIe3.0 speed.

Thunderbolt 3's PCIe3.0 x4 interface (32.4Gbit/s i.e. ~4GByte/s raw interface bandwidth) will be a bottleneck for sequential transfers, instead of the drive's native capability of up to 7.4GB/s read or write. You might still be able to get close to the 2 million IOPS mixed read+write operations this drive claims to be capable of, for small mixed read+write, though. (up to 1.55 million random 4k read IOPS, up to 1.6 million random 4k write IOPS). Also, the low latency guarantees should fully apply, like <6 us for 99% of small requests, and <66 us for 99.999% of small requests.

(I'm not sure if Thunderbolt adds any meaningful amount of extra latency to PCIe transactions beyond the few nanoseconds of light-speed delay from cable length and a few gate delays from muxing. wikichip shows some details of Thunderbolt 3 in Ice Lake, but doesn't go into detail about if/how Thunderbolt encapsulates PCIe packets.)

That drive comes in U.2 and E1.S form factors. U.2 is like a 2.5" laptop / server drive, but with PCIe connectors instead of SATA, intended to slide into a bay in a storage server. Lets just say that in theory you could connect it to Thunderbolt. See also https://nvmexpress.org/new-pcie-form-factor-enables-greater-pcie-ssd-adoption/ re: form factors.

Something you definitely could use with a Thunderbolt -> PCIe slot enclosure is a DapuStor H3100 which comes in HHHL card slot form factor (Half Height Half Length), PCIe3.0 x4. So the numbers you see on this review are using the same interface speed you could get via Thunderbolt. 3528 MB/s read, 2603 MB/s write, 803 kIOPS random read, 250 kIOPS random write.
This is an eTLC NAND flash device, so obviously it's much slower at random write than the Optane using 3D XPoint. And the latency is worse. MacOS isn't on the supported OS list, but it is an NVMe SSD.


Note that sequential bandwidth isn't the only criterion for storage speed. NVMe has some inherent protocol / driver advantages for low latency and high number of small IO operations per second (IOPS), important if you want to use this storage for a database server for example.

Some storage hardware-review sites do benchmarks that test drives in both low and high "queue depth" situations, i.e. number of outstanding requests in parallel. In a single-user desktop situation, it's common that a program won't do other reads until after the data comes back from the current read. (e.g. loading an executable, it can't run and make open system calls until it itself is loaded). So desktop responsiveness is often more correlated with low queue depth IOPS and latency, like QD 1 to 4.

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  • This is not an area I am too familiar with, is something like the Samsung Evo 980 PRO the sort of spec you mention?
    – Mr. Boy
    Jul 26 at 13:20
  • 1
    @Mr.Boy: That's a consumer / prosumer drive, not designed for servers, so no it's not what I was talking about. But its peak / best-case performance is really high as rated by the manufacturer (samsung.com/semiconductor/minisite/ssd/product/consumer/980pro), more than enough to saturate PCIe3.0 x4. And ~1 million IOPS read/write. It's totally a realistic option, unlike the much more expensive enterprise-grade drives that are designed for stuff like database or web serving workloads to keep them busy 24/7, not just run in bursts like normal desktop workloads. Jul 26 at 13:27
  • 1
    @Mr.Boy: On Samsung's web site, note that they break things down into 3 main categories: consumer, portable, and data centre. It's the "data centre" drives that I'm talking about as being somewhat faster even for desktop workloads, at a big price premium. (Like I said, you asked for "fastest" without setting any budget / "worth the price" constraints, so this is an exercise in what's possible, not a shopping suggestion.) Jul 26 at 13:30
  • Thanks Peter that makes sense
    – Mr. Boy
    Jul 26 at 13:45
0

Im keeping this fairly non-technical, so I'm sure someone will be along to quibble with the details. ;-)

Thunderbolt is "lots of different things". It includes PCIe, which is the standard for connecting devices to computers' motherboards. So instead of having a slot on the board, you now have a cable. It also includes DisplayPort video signals, and power.

This is why you can use Thunderbolt to connect to a 'hub' that provides more USB ports, video ports, SD card readers, and Ethernet. It's a motorway for different vehicles.

Thunderbolt 1, 2, 3 (and 4!) are just newer, faster iterations of the technology. (As with USB.)

USB was devised as a general means of connecting devices: mice, keyboards, drives, etc.

Thunderbolt devices will generally be slightly faster than USB ones. And by faster, I mean expensive.

NVMe is a property of the device itself, rather than a 'transport'.

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