Migration Assistant can't find Time Machine backup drive

I'm replacing an older Mac (10.6.8) with a new iMac. I first attempted to restore the backup during install, but even though the Time Machine backups were listed properly, when I selected the newest one it said I couldn't restore it because the source wasn't the same type of computer (it was a Mac Pro).

I then tried to use Migration Assistant, however it doesn't see the drive at all. It just keeps searching. The drive mounts normally in the OS, and disk utility says it's healthy. Is there a way to get Migration Assistant to see this backup drive? Is the only option to copy any files I want off the drive manualy?