SUN tape drive problem

[Jay Roth]

I'm having a problem with my SUN tape drive, and perhaps someone out there
has the solution, or can help.  Many times, when I insert a tape, then try
a "mt" command, the tape can't be "found" (I get "no cartridge in drive..").
Once this happens, according to my unix manuals (and verified by experience),
any mt command to the drive will fail, until the "descriptor is closed."
I can't figure out a way to close the descriptor (sometimes rebooting isn't
good enough).  Any ideas on what the problem is, or how to get around it?