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I'm setting up a new 2019 iMac with an internal Fusion drive. I have an external 1TB Samsung T5 SSD drive that I'd like to use velcroed to the back of the iMac as the main system drive in order to improve performance.

Should I format the SSD to APFS or HFS+?

The iMac has MacOS 10.15 Catalina installed by default.

(I've read some stories of 2 minute wait times after restarts using APFS on external system drives, but I'd like to use the most up to date system if it's right to do so).

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    I have my doubts that the speed of the interface to an external SSD can exceed the speed of an internal Fusion drive. – WGroleau Jan 28 at 18:21
  • Quite a few examples of people doing this online in order to gain performance improvements - not sure but I think the bottleneck might be seek times on fusion drive vs SSD rather than data rate between the drive and main bus? - eg youtube.com/watch?v=BXZan8T9ltI – tomh Jan 28 at 18:26
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    @WGroleau: If it's connected via Thunderbolt, the performance can be surprisingly good! I'm sitting at an iMac 27" 2017 right now, with a Samsung X5 connected via Thunderbolt. According to Blackmagic Disk Speed Test it has almost exactly the same speed as the internal Apple SSD (both about 1200 MB/s Write, 2200 MB/s Read). – DarkDust Jan 29 at 8:05
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Only APFS is supported for running Catalina. There may be some weird workaround for running Catalina on HFS+, but I don't know of any.

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Pedro has already correctly answered your question. This answer addresses your statement of improved performance. The internal fusion drive is basically a SATA HDD and a PCIe SDD. While your 1TB Samsung T5 SSD drive will be faster than the internal HDD, it will not be faster than the internal SDD. The reason for this is the T5 uses a USB 3.1 Gen 2 interface through a USB type C connector.

To compete with the PCIe SDD, you would have to step up to a Samsung X5 SSD drive. This has a Thunderbolt 3 interface though a USB type C connector. This Thunderbolt 3 interface is 4 times faster than a USB 3.1 Gen 2 interface. Both interfaces use the same USB type C ports on the back of your 2019 iMac.

I realize the cost of a X5 is significantly higher than a T5. There are cheaper Thunderbolt 3 SDDs, but the lower price usually means less performance. Some comparisons are given below.

$170 Samsung T5 1 TB USB 3.1 Gen 2 SDD with speeds up to 540 MB/s

$200 VisionTek 1 TB Thunderbolt 3 SSD with speeds up to 1800 MB/s

$409 Samsung X5 1 TB Thunderbolt 3 SSD with speeds up to 2800 MB/s read, 2300 MB/s write

The other issue with a USB SSD is whether or not UASP mode can be enabled on a Mac. Note, the link for the T5 has a disclaimer which states the 540 MB/s is dependent on the ability of the computer enable UASP mode. This issue does not seem to occur when a Thunderbolt 3 interface is used.

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    macOS supports UASP, but in practice, the drive controller (such as that of your SATA-to-USB adapter) or any USB hubs being used may not, so those will be the limiting factors. – Jivan Pal Jan 28 at 23:58
  • @JivanPal: Do you know if there is a way to confirm TRIM is turned on and is being used? Evidently, if TRIM does not occur, then drive will eventually slow down. – David Anderson Jan 29 at 0:08

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