problem with using 'dd' to copy 'tar' tapes

[Steve D. Majewski]

In trying to make a copy of a tar tape with dd, the copies don't seem to
be exactly the same as the originals. 

The original, I can loop thru via: "dd if=/dev/nrst0" or "tar -tvf
/dev/nrst0"  and at the end, the tape is properly positioned at the start
of the next file.

Reading the copy, the tape sometimes ends up positioned at the end of the
current file, rather than the beginning of the next. 

  "mt status" shows: 
    Archive QIC-150 tape drive:
       sense key(0x0)= no sense   residual= 0   retries= 0
       file no= 3   block no= 17506

I assume that this has something to do with padding and some magical
combination of "bs=" and "conv=block(,sync?) will fix it. Does anybody
know the right magic ? ( I assume it is padding of blocks on the tape,
because there doesn't seem to be any OTHER problems, but I'm not sure how
to verify it - I don't have room enough to diff 2 copies on disk. ) 

 [  This is on a Sparcstation I / SUN-OS 4.03 / Archive QIC-150 tape ]

 Steven D. Majewski		University of Virginia Physiology Dept.
 sdm7g at Virginia.EDU		Box 449 Health Sciences Center
 (804)-982-0831			Charlottesville, VA 22908