The Media Recognition System (MRS) gotcha probably bit the user. DDS-1 (90 meter), DDS-2 and DDS-3 drives can read from and write to 60-meter tapes, provided the MRS requirements are met; i.e. either the tape uses the MRS or the drive has been configured (via a dip switch) to disregard MRS. Note that most drives come by default configured to require MRS, so if you have an internal DDS-1 (90 meter), DDS-2 or DDS-3 drive, chances are very good it requires MRS tapes, at least for writing.

So, how do you tell whether a tape uses the MRS? Some tapes will actually have printed on the case somewhere "Media Recognition System", but this is not always true. The thing to look for is the 4-bar symbol (||||) on the case. This always means the tape uses the MRS. My advice is to discard any tapes that do not use MRS unless they are archive tapes you must keep for read purposes. Do you really want to play around with a potentially non-MRS tape when you are trying to take a memory dump or create a CSLT/SLT?

Denys Beauchemin contributed this warning about the new DDS-4 standard:

A DDS-4 drive will be able to read from and write to DDS-3 125-meter tapes and DDS-2 120-meter tapes. It will only be able to read from, not write to DDS-1 90-meter tapes. DDS-4 drives will neither write to nor read from DDS-1 60-meter tapes.

This has nothing to do with MRS. But it has everything to do with the way HP currently distributes FOS, SUBSYS, POWERPATCH and PATCH tapes: on 60-meter DDS-1 tapes. I suspect that by the time the n-class machines come out, if not sooner, the internal drive will be DDS-4. You've been warned.