If by disk drive you mean the storage where you keep multiple virtual tapes, then in terms of physical tape library, your question can be rephrased as follows. "Should I use one physical tape library per box of virtual tapes?". Well, it's entirely up you. It depends on what you are trying to achieve.
Thanks for the quick response. I actually mean: should a virtual tape library be limited to one disk drive? I have four 2-tb eSata disks for backing up and I need Firestreamer to fake out DPM so we can rotate the disks offsite.
Should a tape library be limited to one box of tapes when I have two boxes of tapes? No, I don't think so. Just use a single tape library. You can always add a second one if you need to.