I'm trying to figure out if my customer's backup program is working correctly. They have about 30 gigs of data backing up to a 20/40 tape. If that was 30 gigs of text documents, then it would definitely fit.

But the data is mainly AutoCad files - as they're an architects firm.
So are AutoCad files compressed files? If so, then that would explain why the backup requires two tapes.
If not, then ....