Unable to clear Fusion Drive SSD or reinstall macOS

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permoeng Author

Level 1

8 points

Unable to clear Fusion Drive SSD or reinstall macOS

I have a iMac 27" from 2014 with a 1TB Fusion Drive.

I have now sold the Mac and want to clear the disks for any data and reinstall the OS.

The computer had macOS Big Sur installed.

I couldn't find a good guide to how to delete my data on the computer, and now I'm a bit lost...

I have deleted/formatted the 1 TB HDD and when I wanted to reinstall the OS it didn't find the SSD part of the Fusion Drive to install the OS, only the 1 TB HDD drive came up as an option.

After several attempts the computer still doesn't let me clean the SSD part of the Fusion Drive, and now only OS X Yosemite, which came preinstalled on the computer, is the only OS it will install, but again only on the HDD, not on the SSD.

I have tried to follow these steps: How to fix a split Fusion Drive - Apple Support

but the Terminal prompts doesn't work, I get "couldn't unmount disk" error.

This is how the disk setup looks in Disk Utility

And this is what's listed in Terminal when I type "diskutil list"

iMac 27″

Posted on Dec 4, 2025 12:20 PM

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Answer 1

HWTech

Level 9

68,432 points

Dec 4, 2025 6:07 PM in response to permoeng

Before you do anything, I highly recommend you first create a bootable macOS 11.x Big Sur USB installer so you have options in case you have issues installing macOS with Internet Recovery Mode.

Also, it appears are you booted into the macOS installer for the version of macOS which originally shipped with that Mac. See if you can boot into the online macOS Big Sur installer instead using Command + Option + R. Unfortunately some Macs may only boot to the older online installer regardless of the keys used for booting (I'm assuming you upgraded to macOS 10.13+ at some point).

Try using Disk Utility to perform a simple erase on each physical internal drive which will break the current Fusion Drive setup. If you still get the "unable to unmount" error, then try using Disk Utility to unmount the Fusion Drive itself (disk2 & disk3 in your pictures). If you need to use the command line to unmount the Fusion Drive, then use the following command:

diskutil  unmountDisk  disk2

Afterwards, you should be able to use Disk Utility to perform a simple erase of each physical drive which should then allow you to create a new Fusion Drive setup.

Actually, you should either first enable Filevault on your system and wait for it to finish encrypting before you attempt to erase it so that no one will be able access your personal files even with data recovery software once you perform a simple erase after Filevault is fully enabled.

Or, instead of a simple erase of the internal Hard Drive, you select the "secure erase" option in order to write zeroes to the whole physical drive overwriting everything on it. A simple erase of the SSD is fine due to how SSDs work.

It will take hours to fully encrypt the drive with Filevault or to write zeroes to the whole Hard Drive. The amount of time depends on the speed & health of the Hard Drive.

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