Tar works, dd doesn't, Why?

[Loren Buck Buchanan]

We can write and read back tapes only via tar.  Dd, magtapetools 
(from comp.sources.unix) and my own custom software is unable write 
or read.  The only error message I can get is:

   "I/O error"

Other tape operations such as moving forwards and backwords of files
and records seems to work fine, as does rewind and status (either via
mt or my custom software).  Cpio seemed to write a file out with no
problems, but I am unable to read it back.

What does tar do differently than dd, et. al. that allows it to work?

The software/hardware specifics are:
  IRIX 3.2
  SGI PI 4D/20 (low serial number, but more recent electronics module)
  Pertec FS2001-2 (has a Pertec to SCSI board).

Thanks & B Cing U

Buck
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