I just used SuperDuper to recover my iMac! It worked great. Good job Shirt Pocket software!!
I had update my 2012 iMac from Sierra 10.12.x to High Sierra 10.13.1 All seemed OK at first but then my disk volumes on 2 external drives disappeared at random times. I tried macOS’s disk utility repair process which did not remedy the situation. I online chatted with Apple, the tech suggested I boot from my recovery partition and then reinstall the OS manually via the internet. I didn’t really want to do this, since it’s a generic we-hope-it-works kind of process.
Instead, I got out the external drive with a bootable image I made with the SuperDuper program right before I updated. Yes, I have done this kind of thing before. I worked in engineering/software development/MIS/IT for 4 decades and I learned the hard way that updates are not always improvements.
So, I connected my SuperDuper boot volume and rebooted the iMac, holding the Option key down. This lets you boot from any attached bootable volume. Selecting the SuperDuper volume brought up my 10.12.x system exactly as it was the moment I made the backup. I then ran SuperDuper, copied from the backup to the iMac internal disk. Shut down, disconnected the backup disk, then restarted. My iMac was restored!
Nice. See how important a backup is?