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My 2015 (Model 17,1 w/OS X 10.14 Mojave) iMac was purchased new in 2017. It was a great system until its Fusion Drive failed. After two unsuccessful attempts to repair the Fusion Drive, I am now concerned whether or not this iMac is reparable: Apple declined to repair a "vintage" iMac, and the "repair" performed by a specialist Mac repair shop was an expensive failure.

The repairability issue seems to be largely due to Apple's proprietary SSD designs. For some models, one can find 3rd party replacement SSDs, but for other models, compatible repair parts are not available from any source. For example, OWC - the primary 3rd party source for replacement SSDs for Apple products - does not make an SSD compatible with my iMac Model 17,1.

Some have suggested that a simpler solution to repair a Fusion Drive is simply to replace it with a single, non-Fusion SSD connected to the iMac's internal SATA cabling. This strikes me as an ideal solution - assuming there are no awkward tradeoffs. This question(s) is intended to discover what - if any - tradeoffs are required for the non-Fusion repair option. It's highly likely I do not know what these tradeoffs might be, so I'll list the ones I can think of & ask those providing answers to list any others they're aware of.

Potential tradeoffs & questions for a non-Fusion repair option:

  • Performance: I'm not very concerned about reductions in classic storage drive performance. SATA3 is never going to equal PCI data rates, but the performance limitations I want to overcome are those that require 60 seconds for a login, and 30 second delays in launching an app!

  • Question 1: Will the non-Fusion repair option eliminate the horribly sluggish performance I see now?

  • Question 2: My Time Machine backup was done on the system when it had a Fusion Drive. Will this backup restore to the non-Fusion system?

  • Question 3: Should the SSD portion of the Fusion Drive be removed from the system, should it remain - or does it make any difference?

  • Question 4: Do you know of any other likely tradeoffs or artifacts wrt a non-Fusion system?

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    Personally, I would leave the HDD in the Mac and add a Samsung T5 or T7 USB drive. Install macOS on the Samsung. I did this will a iMac14,3 which never had a internal SSD and has a internal 1TB HDD.
    – David Anderson
    Commented Nov 25, 2022 at 12:17
  • @DavidAnderson: I appreciate the recommendation. The external drive option was suggested on another Q here IIRC. I don't know that I'd want that as a permanent solution, but it would be great as a temporary solution. Is there a good "How To" procedure available?
    – Seamus
    Commented Nov 26, 2022 at 7:10
  • How to? Well you plug the drive into an USB port and install macOS just like as if the drive was an internal SSD. I did not create any type of Fusion arrangement.
    – David Anderson
    Commented Nov 26, 2022 at 7:15
  • @DavidAnderson: I've never actually done that, and I'd like to restore the system from my TM backup.
    – Seamus
    Commented Nov 26, 2022 at 7:20
  • I do not know for sure if you would be able to restore from a TM backup.
    – David Anderson
    Commented Nov 26, 2022 at 7:28

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Much of this is opinion rather than fact (others may do better):

Q1: Depends on the cause of the sluggish performance. My guess is that the cause is not (or not just) the Fusion drive.

Q2: Yes, the backup will restore.

Q3: Opinion: It should not make any difference. But since you have an unknown cause of poor performance, I would get it removed.

Q4: A non-Fusion system will perform more consistently. It has less components to go wrong.

Do note that replacing the HDD with a SATA SSD does require the right temperature sensor.

Regarding OWC, I believe they can supply replacement SSDs. In Australia, I go here https://www.macfixit.com.au/collections/ssd-imac-27-inch-2013-2019. There are SSDs and installation kits.

Edit: read the fine print (as you have done), "Aura Pro X2 is not compatible with iMac17,1 (Late 2015)". But the SSD for HDD should be fine.

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  • AFAIK: OWC supplies SATA SSDs that may work, but about 3X the per TB cost of a Crucial drive. I don't understand how they justify those prices. But their PCI SSDs are a "No Go" for the Model 17,1, per the OWC website, and confirmed by their tech support staff during a telecon.
    – Seamus
    Commented Nov 24, 2022 at 7:07
  • Also curious wrt your comment that you don't feel the issue is entirely due to the Fusion Drive. Can you elaborate on that just a bit?
    – Seamus
    Commented Nov 26, 2022 at 7:12
  • 1) Is/was yours a 1TB fusion (which has 24GB SSD) or 2TB (which has 128GB SSD)? If 1Tb then the 24GB SSD is a definite bottleneck. Not so much for the bigger fusion. 2) From your description, there is some uncertainty about whether your repairs have fixed things or introduced new issues. 3) I don't know enough of the state of your software - an erase and new install might improve things. A new install to an external SSD might clear up your problems - as @David Anderson suggested.
    – Gilby
    Commented Nov 26, 2022 at 9:23
  • It was a 2TB Fusion; 2TB HDD & 120GB SSD; see pre-repair diskutil listing. The post-repair performance has been better than pre-repair, but still has major performance issues - of the same type as pre-repair.
    – Seamus
    Commented Nov 26, 2022 at 10:04

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