The command line to list library slots is:
# omnimm -repository "My Library"
Slot [side] Medium Medium Label (ID)
=========================================================================
1 Data Protector [ML8771L4] Free LTO_5
2 Data Protector foreign [ML8762L4]
3 Data Protector [ML8761L4] ML8761L4
4 Data Protector [ML8767L4] Free LTO_6
5 Data Protector [ML8768L4] Free LTO_7
6 Data Protector [ML8769L4] Default LTO-Ultrium_23
7 Data Protector [ML8772L4] Default LTO-Ultrium_27
8 Data Protector [ML8773L4] ML8773L4
9 Empty
10 Data Protector [ML8775L4] Default LTO-Ultrium_28
You can filter the Unknown tapes from the above output by using Grep command on Unix or Find command on Windows
Regards
Juanjo