I could see the files on the drive – I even moved a couple of folders successfully on the new drive. When I got back from work it looked like it had finished ok. The 2TB transfer was left overnight and during the day. The MyBook Duo used the official Apple Thunderbolt adapter – which is pretty solid, and the WD Elements was plugged into a 3 port USB hub – which is a bit flimsy. Its a 2017 model so has no native USB or Thunderbolt 2 ports – just USB-C, so I had to use adapters to plug in my drives. I formatted it and started with a 2TB backup of my 4TB (2x2TB RAID1 WD MyBook Duo). I recently bought a 4TB Western Digital Elements external drive in the Black Friday sales to add to add to my backup family. In its place would be 'disk0s2.' It is this indented entry that you want to erase.A tale of a brand new disk drive, a 2TB backup, a disk failure and finally a recovery… You might see that it is displayed on the list as: To be sure you do this right, please observe that the drive may appear in the list as ghosted because it is unmounted. Use the device reference in this next command. It will appear similar but not the same as this: /dev/disk0s2. In the output, you should find your Fusion drive listed. At the prompt paste these lines individually pressing RETURN at the end of each line. If that doesn't work then quit Disk Utility and open Terminal from the Utilities' menu. After doing that click on the Unmount button in the toolbar to see if the disk disappears from the Desktop.

Try repairing the disk in Disk Utility using First Aid.

Could not unmount disk usb