My main workstation needed a reboot, so I rebooted it ... and it didn't reboot. It came back with the dreaded "grub-rescue" prompt. And I couldn't work out how to rescue it.
So I decided to reinstall linux (Fedora 30). That didn't go as smoothly as I would have liked.
The first five drives that I tried to install to, were recognised by the HP xw6600, but not by linux. Eventually, I tried a 160gb drive, and that worked. I have no idea why.
So I also connected up the drive that wouldn't boot as the second drive, hoping that I'd be able to copy all the configuration files across.
That worked. I mounted it using mount /dev/mapper/fedora-root temp2 and all the files were there, so I was able to copy them to the newly installed drive. That meant that all my bookmarks, and ftp bookmarks, were preserved!
So why did it fail? I just don't know. Computers fail all the time. Just keep making backups!