mt —
TM-78/TU-78 MASSBUS mag tape interface
mt0 at mba? drive ? tape mu0 at mt0 slave 0
The TM-78/TU-78 combination provides a standard tape drive interface as
  described in mtio(4). Only 1600
  and 6250 BPI are supported; the TU-78 runs at 125 IPS and autoloads tapes.
  - mu%d: no write ring.
- An attempt was made to write on the tape drive when no write ring was
      present; this message is written on the terminal of the user who tried to
      access the tape.
- mu%d: not online.
- An attempt was made to access the tape while it was offline; this message
      is written on the terminal of the user who tried to access the tape.
- mu%d: can't change density in mid-tape.
- An attempt was made to write on a tape at a different density than is
      already recorded on the tape. This message is written on the terminal of
      the user who tried to switch the density.
- mu%d: hard error bn%d mbsr=%b er=%x ds=%b.
- A tape error occurred at block bn; the mt error register
      and drive status register are printed in octal with the bits symbolically
      decoded. Any error is fatal on non-raw tape; when possible the driver will
      have retried the operation which failed several times before reporting the
      error.
- mu%d: blank tape.
- An attempt was made to read a blank tape (a tape without even end-of-file
      marks).
- mu%d: offline.
- During an i/o operation the device was set offline. If a non-raw tape was
      used in the access it is closed.
Themt driver appeared in
  4.1BSD.
If a physical error (non-data) occurs, mt may hang
  ungracefully.
Because 800 BPI tapes are not supported, the numbering of minor
    devices is inconsistent with triple-density tape units. Unit 0 is drive 0,
    1600 BPI.