Hi Joh,
I had a similar situation and this is how I handle it, I have 1 Drive, and a HP StorageWorks 6030 (28 tapes).
I first go under 'Robotic Libraries' and under that library I create several partitions. Each of these partitions I have assigned multiple slots. Say for a 600 gb job I have 2 slots x 400 gb tapes.
Then under the 'Job setup' tab I edit the job I want and under 'Destination>Device and Media' I set the Device to that particular partition. This ensures that it won't go and write outside those tapes on top of the Media Set protection.
While I do this on fulls and erase the media before hand to ensure I have enough space and no old data. This will select a tape from that partition, fill a tape and roll over to the next tape in that partition.
Hope that helps your situation
Harry