[Dump-users] Cannot open output<?>

[<cco...@sw...>]

When running a dump:

# /sbin/dump 0bdsf 64 61000 12676 /dev/nst0 /
  DUMP: Date of this level 0 dump: Mon Feb 28 09:38:51 2000
  DUMP: Date of last level 0 dump: the epoch
  DUMP: Dumping /dev/sda1 (/) to /dev/nst0
  DUMP: Label: none
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 64368 tape blocks on 0.01 tape(s).
  DUMP: Cannot open output "/dev/nst0".
  DUMP: Do you want to retry the open?: ("yes" or "no") no
  DUMP: The ENTIRE dump is aborted.

I'm curious to know what the "Cannot open output..." line means, as well as why dumps would run sporadically.

Tapes which have been successfully dumped in the past are restore-able, but when I try to run a dump, dump fails as above.

Also, can anyone point me to the algorithm for determining the magic incantations for blocksize, length and data density?

Thanks in advance,

chris couples