Replace Hard Drive in iMac with Fusion
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.