How to erase an APFS-formatted Fusion drive

I’m running Mojave (10.14.6) on an iMac. Disk Utility has found a problem on my APFS-formatted Fusion drive that it can’t fix. I want to erase my Fusion drive, reinstall Mojave (perform a clean install), and then use Migration Assistant to restore from my recent backup (SuperDuper! clone).

When I boot to macOS recovery, I’ve run Terminal and tried to execute “diskutil resetFusion” but for some unknown reason that command is not seeing my Fusion drive as something it can “reset”. I’ve also booted to an external drive and tried the same command in Terminal with the same result. I’ve spent at least three hours chatting with Apple support but they have yet to provide any solution that works.

Does anyone know the procedure to erase an APFS-formatted Fusion drive, hopefully without splitting the drive, or causing any other undesirable side effects? If I had a solution where the Fusion drive was reformatting as HFS+, I could live with that.

When you boot in recovery mode, can you launch Disk Utility and use that to nuke your Fusion drive?

Back in 2015, I got into a situation where I needed to recreate a fusion drive on a new iMac and couldn’t use Disk Utility (long story but off the point). So I searched for a cookbook that even I could follow and bookmarked it to my Safari reading list. So, here is the cookbook I actually used to create a clean HFS+ Fusion Drive between the internal SSD and Hard Drive on my iMa and have used it ever since. Hopefully, this can help.

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It turns out that you can (at least the Macintosh HD portion of the Fusion drive). In Disk Utility, if I select View: Show All Devices, “Fusion Drive” was listed first, then below it “Container disk2”, and then below that, “Macintosh HD”. In my lengthy support chat with Apple, I was told to erase the top entry, which is “Fusion Drive”, but when this device is selected the Erase button becomes disabled. That’s what confused me.

Alan - that is a very interesting link, which I’ll save for future reference - thanks.

It turns out I received a reply midday from an email I sent Glenn Fleishman last night. He suggested I go to https://blog.macsales.com/52253-how-to-upgrade-install-macos-catalina-safely/ which had relatively simple directions for performing a clean install for Catalina, even though I used it for Mojave, and it worked wonderfully.

I have been through this performance more times than I care to mention, I have recreated the fusion drive at least three times using terminal, all very simple and it shows as a fusion drive in storage and in disk utility though unlike you I have fusion drive then container disk then Macintosh HD and a further Macintosh HD. I reinstalled Catalina but the boot time and general performance is so slow as to drive anyone nuts.
I would love to fix this but it seems the only solution would be to replace the fusion drive, in the meantime I run off an external 500GB SSD. Any help or thoughts would be deeply appreciated.

Not the first place you’d think to look for help with this, but Other World Computing sells flash storage devices that fit into the same slot in iMacs that’s occupied by the flash ram part of the Fusion drive. Supposedly these dramatically upgrade the performance of the computer, assuming whomever rips the screen off the monitor can do the modification without ruining the iMac (I have all the electronic bits, but I’ve been too chicken to start the project). They have installation videos on their site, and there are instructions there about how to split the Fusion drive (and, I think, recreate it). They also have VERY friendly and knowledgeable support staff who may well be willing and able to address this issue on the phone.

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I have been receiving their newsletter for years and they do seem to have good products and people however on watching the video for replacing the fusion drive with an all SSD it looked terrifyingly difficult and fraught with opportunities to destroy my Mac. I just have to assume that the fusion drive is at the end of its useful life and move on unless anyone else has a comment, I will try the link offered by an earlier poster, we shall see but perhaps not overcome. Congratulations to Tidbits I think I was among the early subscribers.

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Thanks for reading TidBITS for so long! Yeah, opening a modern iMac is not for the faint of heart.

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Re-reading this, I’m wondering if you really DID re-create your Fusion drive. After all, the total storage space is determined by the rotating platter, and the SSD portion is basically a rapid-access cache, perhaps it’s actually doing NOTHING.

Hi
I’m sorry I don’t know how to address you, I have long held the same view but I assure you I followed the steps on the Apple forum to the letter not once but several times, your theory the the SSD is doing nothing is I believe totally correct but the problem is why. There is no damage to the drive I have run repair from Disk Utility without any problems showing up if you have any thoughts that you think might help I would be most grateful if you could share. I continue to run the 27in off an external 500Gb SSD which is OK but I really would like to get back to what it should be.

Best

Mike

PS I trust you are all safe and well, here in Hong Kong we had our second day this week without a new case so fingers crossed maybe we will soon be able to get back to normal.

As I mentioned elsewhere, my own iMac is boxed up to be buffed up when I get the courage to open it up (probably more dangerous to my computer’s life than living in Georgia today might be to my own DNA-derived existence, but that’s another story).

Assuming that your efforts to re-create the Fusion between SSD and rotating platter into a logical single volume with smart management of frequently used data, I tried to find out:

  1. Whether you actually could format that SSD as a single separate volume mountable on the Mac desktop, perhaps containing the system, perhaps used for data

  2. Just how large would that independent volume be?

The answers were:

I don’t know (partly because my iMac is boxed up)

and

The answer is available, at (of all places) TidBits, to wit: