I have used GNU ddrescue to copy a crashed WD external disk over to an identical one. ddrescue told me that 9 MB out of 2TB could not be rescued, so I think that's pretty good.
Anyway, being a Mac disk, I connected the new disk to a Mac and ran Disk tools, which told me that the filesystem needed repairs. When clicking Repair, it first successfully (or so it says) repairs the filesystem and then gets stuck on something like
Updating Boot Support Partitions for the volume as required
I have no idea what to make of this. How long should I wait? Can I cancel it safely?