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We have a Late 2012 iMac in the shop that needs the 3TB hard drive replaced. Turns out this model has a Fusion drive with an SSD that is on the backside of the logic board.

The drive has been swapped (new WD Blue 3TB), but we are having issues getting the Fusion setup, and getting OS Sierra loaded. I have tried researching the Fusion drive and what needs to be done, but all I can find is people trying to replace the HDD with an SSD, and other "projects".

When we go to Disk Utility from a Sierra flash drive, it does not give me any options to rebuild the Fusion drive or anything like that. If I plug up the original HDD to another Mac, it does tell me it needs to rebuild it, so that confirms it is a Fusion drive.

How do we properly replace the HDD, and either get the system booting as-is (is the OS on the SSD?), or reload OS Sierra on it? We are currently working on doing a TimeMachine backup on the system, but it is about 2.2TB of data...

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A Fusion Drive is a composite of two physical drives: a fast but small SSD and a slower but larger HDD. Each of the physical drives contains three partitions: an EFI partition, a so-called physical volume and a special volume. The special volume on the HDD usually is the Recovery HD.

The two physical volumes are part of a Logical Volume Group. Creating a default Fusion Drive means building one Logical Volume in the LVG spanning both physical volumes. The Logical Volume is the one mounted to root (and if enabled - the volume visible on the desktop).

A file stored on the Logical Volume may reside on the SSD as well as on the HDD. System files may also reside on both drives. Usually an algorithm ensures that system files are stored on the faster SSD but there is no guarantee.

Removing one of the physical drives means removing one of the physical volumes and therefore tearing the LVG as well as the LV apart. After destroying the LVG/LV the file system as well as system files or other files are corrupted and the remnants won't be bootable.


To replace either of the drives, backup your exisiting volume(s) first with Time Machine. Then check whether you can boot to Internet Recovery Mode or create a bootable thumb drive and also try to boot to it.

Shutdown your Mac and replace one (or both) of the old drives. Then boot to Internet Recovery Mode or the bootable thumb drive.

  • Open Disk Utility and completely erase both disks (each to one volume/GPT/Journaled HFS and assign names e.g. "fusion1" and "fusion2" but not "Macintosh HD"). Don't erase your thumb drive or a drive visible called Boot OS X
  • Open Terminal in the menubar Utilities > Terminal and get the device identifiers of the two hard drives by entering diskutil list. Booted to Internet Recovery Mode you will get a list of 14-16 drives - only two of them are your hard drives. Simply check the sizes. The drives related to Internet Recovery Mode have a size of 1.2 GB or smaller!
  • Create a new LVG and LV (here I assume the 120 GB SSD has the disk identifier disk0 and the 3 TB HDD disk1 - they might be different though!):

    diskutil cs create Fusion disk0 disk1
    diskutil cs createVolume lvgUUID jhfs+ "Macintosh HD" 100% #replace lvgUUID with the UUID shown in the output of the previous command
    
  • verify the Fusion Drive with:

    diskutil verifyDisk disk0 # use one of the disk identifiers found previously 
    
  • Enter exit to quit Terminal.app
  • attach the Time Machine backup drive and in the Utilities windows choose Restore from Time Machine Backup > Select backup source > ...choose the drive and restore it to your Macintosh HD volume.
  • After the restore task has finished successfully, boot your Mac normally. Check if a RecoveryHD is present by opening Terminal and entering diskutil list.
  • When the RecoveryHD is missing, simply reinstall Sierra with the latest Sierra installer downloaded from the App Store - which shouldn't affect your data but creates an additional RecoveryHD - or use the Recovery Partition Creator 4.
  • Excellent answer! :) I took the liberty of adding a link to Recovery Partition Creator 4 (it looked like you had tried to add one, bit it was missing for some reason). My apologies if I've linked to the wrong page, but I thought I'd mention it so you could double-check! – Monomeeth Mar 28 '17 at 22:52
  • @Monomeeth ;-) I copied some parts from another similar answer and forgot to add the link... thanx – klanomath Mar 28 '17 at 22:54
  • @klanomath Thanks for the quick and comprehensive answer! I will mark it as the solution once we get this going, in-case I have more questions to be added. ;) We're currently still trying to do the TimeMachine backup, seems this original drive might have been failing (thus the replacement), and it's just chugging along... – Demonslay335 Mar 29 '17 at 13:56
  • @klanomath Would the system run terribly slow in the recovery environment because of the "broken" Fusion? Like, the "Hello" in different languages on the selection screen lags noticeably. We're suspecting this system has more issues than just the HDD based on how terrible it runs when booted to a flash drive. – Demonslay335 Apr 3 '17 at 14:03
  • @Demonslay335 If you've booted to Internet Recovery Mode, the (recovery) system is entirely loaded to RAM and should be fast & responsive. Booted to a thumb drive it may be slow though (depends on the thumb drive). My SANDisk Extreme USB3 thumb drive (real r/w ~80-120 MB/s) with a full! 10.9 system boots in maybe 70 seconds now. It was slower (but not much) at the first boots. Slow ("cheap") USB2 thumb drives may take much longer though. – klanomath Apr 3 '17 at 14:21

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