Hi,
given the MA host is on 2003 and thus a little older, it's probably just the LTO tape driver that doesn't recognize LTO6 yet. Upgrade that driver to the latest version from HP, make sure the drive pops up in device manager, and rediscover (or manually add) in DP.
BTW, why not plug the LTO6 FC drive into the MSL2024? I would consider a 1/8 loader a downgrade when you came from 24 slots. Of course, if the MSL is from the LTO3 or 4 era, you probably still need it to access the old media for some time to come, and with the 1/8 you reduce the temptation to leave all your LTO6 media in the library (less work with vaulting, but means losing all the tapes together with the backup server in case of a catastrophic event). If the MA is on 2003, is it really powerful enough to feed an LTO6 drive, though?
HTH,
An"2nd thoughts"dre.