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A2115 / 2019 / Processors from 3.0 GHz 6-core i5, up to 3.6 GHz 8-core i9. Released March 19, 2019.

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Getting rid of the Fusion Drive for a WD SN770 NVMe SSD Blade


I want to upgrade a 27” 5k 2019 iMac that has a fusion drive in it.

With the Big Sur update, it became horribly slow. The fusion drive was probably a poor choice from the beginning. It’s my mom’s mac and she was scared she wouldn’t have enough storage with a 250GB SSD, even though I explained to her that internal SSD + external thunderbolt drives were the best choice.

So I guess I have two options:

A/ Removing both the fusion drive SSD and the drive and putting a Sata 2.5” SSD in place of the original HDD as the main internal drive. I think 2019 iMacs have 2,5 format HDDs, so I suppose putting a 2,5 SSD wouldn’t require any adapter. It’s the choice I like the least as NVMe M2 SSD has dropped in price and are faster than SATA SSDs.

B/ Removing both the fusion drive SSD and replacing it with an NVMe M2 SSD (and also removing temporarily or permanently the HDD as it is linked in a fusion drive setup, and needs at least to be “unlinked”. I think I might just dump it all together for convenience.”

I like this option better, but I have several questions:

1/ Regarding data migration, I read here that since the system partition is APFS, it’s not possible anymore to just clone the whole partition with Carbon Copy Clone to the new NVMe M2 SDD plugged in externally in some kind of enclosure. This puzzles me because CCC states that it can work with APFS and I know for a fact that until Mojave at least, it did a good job cloning not only the data but a recovery partition as well. I’m quite reluctant to use the data migration tool provided by Apple, which relies on Time machine if I’m not mistaken because I had issues with it in the past…

@danj, you seem to know a lot about this, can you please confirm it to be that using CCC for this isn't possible anymore, given that the version of Mac OS used here is Big Sur?

2/ Regarding the hardware, I saw videos of guys doing basically this, but it seems that an adapter is needed. Which ones are known to be working for this upgrade?

3/ Also, regarding the M.2 SSD that I would use for this, I was thinking about a Western Digital SN770 NVMe because I read on a Hackintosh forum that these were always compatible with recent Macs, as WD is now the main company that Apple relies for its production concerning SSDs. Is there maybe a more natural choice for this upgrade?

Thank you in advance for your help :)

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I use Apples TimeMachine and Migration Assistant. Apple has not released the deeper details on how to properly ‘Clone’ APFS. What you are doing with CCC is the older block style of clone copying Which is a bit of a crapshoot! They assume all of the blocks are clean and none are an located spare block from the reserve. Then to add to it if you enabled encryption all bets are off!

I haven’t used any cloning software on startup drives for years as I want to do the job as quickly and as efficiently as possible as time is money and I don’t want to deal with the mess afterwards.

But just like playing Russian roulette are you willing to play?

FYI - A Fusion Drive is physically two drives the blade SSD (Caching) and the SATA HDD drive (Data).

Now the next point I don’t recommend using M.2 SSD’s in iMacs as they are fussy! Given how deep you need to go in, it’s best to stick with either a real Apple or OWC SSD drive they work and are reliable and yes a bit more cost.

Frankly, I don’t think your Mom is pushing the system that hard, a SATA SSD will more than likely meet her needs. So get a SATA to USB or Thunderbolt adapter/drive case to hold the new drive and just run Migration Assistant to migrate everything over to the new drive, check it by booting up under it and then pull both drives and install the new drive - Done!

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Thank you so much @danj for your answer!

From my perspective and experience, I was under the impression that using Time Machine / Migration assistant was more of a Russian roulette situation than cloning, but I never used encryption. I also admit that I tried Migration just once and I don't even remember well what was the problem I had with it :) ! Now I understand better why cloning isn't that good, because of the way it copies all blocks, "clean" or not

Ok, so you basically recommend option A! It seems easier and less uncertain, but I find it quite unfortunate not to take advantage of the speed of an NVMe drive. Can I ask you what you mean by "fussy"?

This guy seems to have done an NVME M2 SSD upgrade on the same iMac model, using an adapter, but of course, it's impossible to say whether I would have exactly the same output with the iMac I'm dealing with or not...

Thank you again, I greatly appreciate it!


The strange thing is my mom is not pushing the system at all! There seems to be a problem related with fusion drive (IMHO). The partitions seem ok in disk utility, there is enough free disk space, memory is barely used, CPU usage is very low as she mostly uses MS word and browses the internet (I know, a 27" 5k 2019 iMac is a little bit of overkill for this, but she got it as a retirement gift from her job). But now, it's almost impossible to use this mac, as it's lagging for literally anything! Just clicking on the apple menu takes 15 seconds to launch it. Launching apps takes ages... I suppose it's more of a software problem as it became very slow after the Big Sur update, but since this mac could also use a big SSD as a main drive, I figured I might as well kill two birds with one stone and ditch the Fusion drive (and by that I mean the two parts that are composing it).


@rafaeru - Fussy it’s just that sometimes the adapter shifts and/or the house keeping tasks fail to work so the adapter and drive are killed.

If you had a MacBook Pro then that’s a lot easier to get to.

Again you are playing Russian roulette! I don’t play Russian roulette! the deck is stacked against you.


@danj I just did, it found no problem. (code: ADP000). The mac is still as slow as before though.


BUT! It then automatically launched on recovery mode. I went to disk utility and ran the SOS script on the two partitions in found. I'm pretty sure I already did it from the booted MacOS partition and I thought I did it in recovery mode too.

It ran for some time and then I restarted the mac and the problem is gone! Thank you so much, I would never have try this again without this diagnostics startup thing!

It's still quite slow to launch apps compared to any other mac with similar specs that has an SSD as a main drive. It's no surprise as Fusion drives are really HDD synced with a small SSD that acts as buffer/cache to speed things up. However, it's not abnormally slow anymore!

I'm gonna think a little bit about whether it's really useful to try and upgrade this machine, but if I do, I'll follow your advice and won't try anything crazy.

Thank you for your great help :)


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@rafaeru - have you run the onboard diagnostics to see if an error message popped? I would do that first before assuming you have a drive issue given this added details. Restart the system and press the ''’D’'’ key to enter. Let us know what you discover.

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