At 12:02 PM 12/19/00 -0800, you wrote:
>TK70 media when bulk erased becomes TK50 media.

Not quite, TK70 media and TK50 media are identical in terms of their specs 
(magnetic coercivity etc) once a blank tape is written to by either a TK50 
or a TK70 it becomes that kind of tape. The compatibility at that point is:

         Drive
media   TK50    TK70
TK50    R/W     RO
TK70     --     R/W

--Chuck