I took the decision that I would install Mac OS X on the external drive and then from there retrieve the files from the iMac hard drive.
I could then either reformat the iMac HDD and reinstall OS X or use the external drive as the main drive. Less keen on the latter option as the drive would be significantly slower than the fusion drive on the iMac.
I am hoping that it is a software issue with the drive and not a physical problem. Regardless I did want to take off the data and try a fresh install.
If that didn’t work, then it might be a trip to the Apple Genius Bar, but I didn’t fancy spending £300 on a replacement fusion drive, especially as the computer is now eight years old and having done this already before.
So I connected the drive to the iMac by the included USB-C to USB 3.0 cable and started the iMac off in recovery mode. I did try and install OS X onto the drive, but that wasn’t going to work as the default file structure on the drive when it shipped was FAT. So I started Disk Utility and formatted the drive to APFS ready for installing Mac OS X. This all worked and went to install OS X.
This also worked and I was feeling quite pleased, however the real challenge was going to be was moving the data and documents over from the iMac hard drive to the external drive.
However I had forgotten about the Migration Assistant, a tool I had never used. Usually when I buy a new Mac I like to start from scratch and only install the apps I know I am going to use.
However this time I thought the Migration Assistant would be a better choice.
So I said yes in the OS X install screen and selected the internal iMac drive and selected all the files I wanted migrated and then waited….
As I was told it was going to take fifteen hours…
So I waited…