The error you are getting is a media incompatibility error. You need to first go into the drive menu,and make a data tape. This will set the tape up for use with the format you are using, (either 9840 "A" or "B"). If you are trying to retrieve data from these tapes, it is likely that they are written on a "B" drive and trying to read them on an "A" drive. The "B" drive can read "A" tapes, but "A" drives cannot read "B" tapes.
Good Luck
Andy