Damn, I'm running out of ideas.
Do all of your imported tapes go directly to Imported media pool or sometimes you import media written by 3-party vendor and therefore some cassettes appear in Unrecognized media pool?
Actually, in both cases you can do the following:
1) Get tapes (if any is present) in Unrecognized and Imported media pools.
2) Assign result to variable
3) Import tapes
4) Get tapes (if any is present) in Unrecognized and Imported media pools.
5) Assign result to variable
6) Compare two variable, a difference will be represented by newly imported tapes