Ok, I finally figured this out. The problem is that the old and new disks both had the same name (MyBigDisk) and time machine was getting consused. To fix
- Complete the restoration of the drive via timemachine
- Rename the drive (on Leopard in finder under devices ctrl-click / right-click on the name and rename)
- Go into time machine preferences and remove the drive from the exclusion list
- Perform a Time Machine backup
This will re-backup the freshly restored external drive but this isn't a big issue as older files will be deleted from Time Machine as necessary.
Restoring files on the external disk from before the failure: Now the drive is renamed the older backups seem to have disappeared. Actually, they are there, just tricky to find. To find an old file on the old named disk:
- Go into finder and make sure show path bar is selected
- Go to you home directory in finder
- Enter Time Machine
- Select the date you want to go back to
- In the path bar find the directory with the name like 2011-11-12-202011, click on folder
- The old drive will be in that directory - use the normal restore process
Why is it broken
My guess is that to avoid accidentally backing up the wrong disk (after all every WD My Book is called MyBook when plugged in) they use the UUID (Universally Unique ID) to identify the drive. But, we see the MyBook name in Finder and Time Machine so they need to link the UUID to the MyBook name. This is a one shot, once done, never undone.
So, now when it tries to check that link with the new disk it will have a different UUID (that's the UU bit!) and complains. Renaming that disk breaks the link (there's no MyBook now, so no link to check)
