HP-UX

Here's what I'm getting:
# lssf /dev/rmt/c1t4d0D1600
tape2 card instance 1 SCSI target 4 SCSI LUN 0 at&t 1600 bpi 9 track at addr
10/4/8.4.0 /dev/rmt/c1t4d0D1600

and

# lssf /dev/rmt/c1t4d0wD1600b
tape2 card instance 1 SCSI target 4 SCSI LUN 0 berkeley wait 1600 bpi 9 track at
address 10/4/8.4.0 /dev/rmt/c1t4d0wD1600b

I don't see anything wrong with your configuration. Your special files are right. You have the right driver. The kernel can see the 7980.

Pull the drawer out and watch the tape. Does it move? Can you unload the tape with "mt offl"? Have you tried another tape?

I'm pretty sure that you should be able to read a 6250 tape with a 1600 device file. But I don't see an explicit statement to that effect. You might try making a 6250 device file and see if that works any better.
mksf -v -C tape -I 1 -b D6250 -n
should do it.

The only other thing that I suggest is to check for patches to the tape2 driver.