(snip)..our information is that TSM is smart enough to mount LTO media only in 
drives appropriate to the operation (so LTO2 media will only mount in an LTO4 
drive if it's being read, not written;
LTO4 media will never be mounted in an LTO2 drive; etc.) So if you were to add 
LTO3 media to that library down the road, TSM would access it only through the 
LTO3 drives (and if all the LTO3 drives were being used with LTO2 media while 
an LTO2 drive sits idle, too
bad.) 
 
Can you tell me where you got that information?
Seems to me that coflicts with the statement in IBM technote 1155662.  
"However, mixing LTO-1 and LTO-3 is not recommended because LTO-3 drive can 
read but cannot write to LTO-1 media".
 
If you have LTO2 carts checked in to the library, and LTO4 drives, what is to 
keep TSM from trying to mount one of the LTO2 carts in the LTO4 drive for WRITE?