i've a veeam v7 (last patch) and i've installed a IBM TS3100 library.
After have installed IBM drivers ("non_exclusive" mode), in the Windows device manager, there are two drives IBM and the Media changer. No problem.
With Veeam, the two drives appear in GUI, but no TAPE...
There are many tape in drives (not empty).
With the soft "diagnostic tool IBM", i tested two drives (interface Windows<->TS3100) and no problem...
Tape partitioning is library functionality allowing you to split a library into different "partitions", which might be then presented as separate devices to backup software. But it doesn't matter, since you don't have any of those configured in your environment.
Anyway, we haven't seen similar problems before, so, please open a ticket with our support team and let them investigate it directly. Once a case is opened, kindly, provide its number here.
The most frequent cause of tape issues that we've seen so far is the usage of improper tape drivers, indeed. But I'm glad to hear that you've nailed it. Thanks.