Restoring a corrupted tape

[adiro at math.tau.ac.il]

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We have a problem with a corrupted 1/4 inch cartridge that holds important
data. This data was mounted on tape with the help of the unix command "tar
cv". Later we tried to extract this data since this was the only backup we
had for this data. However, instead of typing "tar xv /our/data" we typed
"tar cv /our/data". The tape was not on a write protect status, so the
result was that that last tar command has mounted an empty (since
/our/data did not exist on the file syst the tar puts an "end_of_file"
mark. This means that although our data is still on the tape (the part we
really need is way beyond the start of the tape) we cannot reach it
because the tape drive cannot go beyond the "end_of_file" mark.  We tried
playing with the following commands tar, mt, dd, bru, mtio but with no
success. It might be that we didn't use the correct smart options.  If you
had this problem before or have any idea of what we can do to save this
data please tell us. We are willing to try everything.  We even tried (on
a sample tape) to see if we can physically cut a bad piece from the tape
and connect the two end, but it seems that this action destroys the
structure of the blocks on the tape.