How do I read a bad tape (tar)?

[Steve Simmons]

olson at anchor.esd.sgi.com (Dave Olson) writes:

>In <1991Apr8.231456.391 at gpu.utcs.utoronto.ca> topix at gpu.utcs.utoronto.ca (R. Munroe) writes:

>| I am having trouble extracting some important data from a cartridge tape.
>| I used tar and now can not retrieve the data.  The tape drive whirrs and
>| clicks a couple of times and then the system tells me to insert the next
>| tape and press <CR> to continue.  I know the data is on the tape - it justs
>| seems that there is a little snag that makes the system think it's
>| reached the end of the archive.  Question - is there anyway way of retrieving
>| the data (whatever is left of it) by either:
>| 
>| 1.  Some unix command that will read past the bogus 'end of archive'.
>| 
>| or
>| 
>| 2.  Are there any companies that specialize in data recovery that could help.
>| 

>Unless you are getting kernel messages on the console or in SYSLOG, odds
>are VERY high that someone overwrote the beginning of the tape, perhaps
>by doing a 'tar c' with no args (fixed in the next release to be a
>no op, instead of trashing the tape), when they meant 'tar t'.

>If so, there is no hope.  The firmware on the drives refuse to read
>past the EOD marker.  Besides, it erased a part of each track when
>the the 'tar c', or whatever was done.  This is one of thoese FAQ
>that shows up in all the comp groups from time to time.  Remember,
>the write protect switch is your friend!

Hmmm...some help might be possible here.  First, one needs to trick
the drive into reading past the EOT (not EOD) marker.  This *can* be
done with some drives (I've done it), as long as one is careful.  An
EOT marker is two tape marks.  So *if* the conjecture about a small
tar blotzing the head of a large one is true, one could do so by doing

   mt -f <no_rewind_tape> fsf 1 ; mt -f <no_rewind_tape> fsf 1

Now the tape is positioned past the EOT mark.  Use GNU tar to read the
damaged data.
-- 
 "FACT: less than 10% of the psychiatrists in the US are actually
  practicing cannibals."  Rod Johnson