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I have an iMac 5k, late 2015, with a Fusion Drive consisting of a 2TB hard disc and a 128GB SSD. I want to upgrade the SSD by buying a third-party one.

What specs should the third-party SSD have to fit in and work with the iMac? NVMe? PCIe? mSATA? M2?

There are two parts to this: it needs to have the right physical connector that fits into the Mac motherboard, and it needs to speak a protocol the Mac speaks. What are the requirements for each?

If I choose an SSD that speaks the SATA protocol, will it still work with my Mac, or must I get a PCIe one? I'm not worried about a slight difference in speed, since both are much faster than a spinning hard disc.

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Your model iMac has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD.

For best compatibility I suggest you purchase your SSD from a reputable retailer. For example, the following are some well-regarded sites:

By buying from a reputable store you'll be sure to get an SSD fit for purpose, regardless of manufacturer. Being reputable means they're only going to sell something that works and they'll stand behind the product they sell, regardless of the brand.

Also, many sites offer an easy interface to ensure you select the right SSD for your particular Mac.

  • Most of these don't sell in India, unfortunately, or have high fees and delays. If I buy an SSD with a SATA connector, I'll have to disconnect the hard disc in the machine, which I don't want to do. That means an SSD with a PCIe connector. Right? – Vaddadi Kartick Dec 9 '16 at 11:32
  • Yes, the SSD uses a PCIe connector (PCIe 2.0 x4 NVMexpress interface to be exact). – Monomeeth Dec 9 '16 at 11:40
  • To be clear, the iMac doesn't have two SATA ports, does it? If it does, I could buy a SATA SSD without removing the hard disc. – Vaddadi Kartick Dec 11 '16 at 7:02
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    Not in the case of your particular iMac. The 27" 5K late-2015 model has one 6.0 Gbps Serial ATA (SATA) interface and one 8.0 GT/s PCIe x4 interface. – Monomeeth Dec 11 '16 at 7:55
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I suggest that you also look at external storage solutions as a much simpler and cheaper alternative, with minimal performance sacrifices.

While my search is by no means exhaustive, I could not find instances of people installing a third-party PCIe SSD in the iMac 5K Late 2015. None of the websites Monomeeth suggested sells a PCIe SSD for the iMac 5K Late 2015, either. Unless compatibility is confirmed, I'd rather not spend money on a PCIe NVMe SSD (expensive), take my iMac apart (difficult and risky), only to find out that it won't work.

Even if there exists such products, they might be Apple proprietary and carry a hefty price premium. In contrast, USB 3.0 external enclosure and SATA SSD are both competitive markets. You have a broad range of products to choose from with wide availability.

If it narrows down to SATA SSDs, the performance difference between internal and external via USB 3.0 becomes very small. You are either limited by SATA 3.0 (6Gbps) or USB 3.0 (5Gbps). In return, you get a much easier upgrade. As you are probably aware of, the iMac 5K Late 2015 disassembly and reassembly is very difficult. I consider it a big plus not having to take an iMac 5K apart and put it back together, although you might enjoy the process and consider it its own reward.

If you have the budget and can find the right PCIe NVMe SSD, go for it. If not, a high quality USB 3.0 SATA enclosure and a good SATA SSD e.g. 850 EVO/PRO will make you just as happy while saving you a lot of time and trouble.

  • I've found enclosures to be nowhere as reliable as internal storage, so I hesitate to use them for the boot disk, but I take your other points, so I've upvoted your answer. Thanks. – Vaddadi Kartick Dec 19 '16 at 13:45

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