Are you doing these restores using the GUI or the command line client? My
experience has been that the GUI will minimize tape mounts and tape
repositioning for a single stream of restore requests. Running a single
instance of the GUI client to restore all 39 file systems might get the
restore done in significantly less time. Ideally, one would like to run three
concurrent streams with each reading from one of the three tapes. I think I
could figure out how to do that if the initial backups had gone directly to
tape, by using various query commands to determine the order in which file
systems were backed up and which files fell at the boundaries between tapes.
Offhand I don't know of a way to determine the order in which file systems
were migrated.