Fixing an unrecognised Fusion Drive
I upgraded my Late 2014 iMac to El Capitan, assuming that it would run faster and smoother. This has not been the case, so I tried to roll back to Yosemite. Something happened during this process and now I am marooned, with neither version working!
I used recovery mode to erase my 1TB Fusion Drive, which worked fine, but the drive doesn’t show up in the OS X installer. In Disk Utility, the drive looks normal but all the options on the Partition tab are greyed out. Is the disk broken? Help! Carol Wojewodzki
Not broken, but unpartitioned, I think. A Fusion Drive is actually two drives: a hard disk and flash storage, treated as a unified volume by OS X, but I’ve seen cases where this breaks down and so neither drive is recognised.
In Disk Utility, where you see the name of the drive, you should also see a line saying Macintosh HD indented below it. The former refers to the Fusion Drive, the latter to the partition on it. If you only see the former, the drive isn’t partitioned properly. To fix it, we will have to trick Disk Utility into repairing it, by deliberately breaking apart the Fusion Drive.
Disconnect all other storage, then start up in Internet Recovery mode (by holding å+ç+r at the startup chime). Choose Utilities > Terminal. Enter diskutil cs list and press ® . The long string of numbers and letters next to Logical Volume Group is the Fusion Drive’s UUID. Select it and copy it to the Clipboard. Now enter diskutil cs
delete then a space, paste the UUID and press ® . Return to Disk Utility. It will recognise that the drives aren’t unified as a Fusion Drive and offer to fix it. Allow it. You should then be able to reinstall Yosemite.