Re: GNU dd and GNU tar See Tape Differently.

[Paul Eggert]

Here's one theory.  tar stops reading when it sees two blocks of
zeros.  Your tape driver has a bug (or at least an incompatibility
with the rest of the world): if a program reads exactly to the end of
a file, but does not attempt to read the first byte past the end of
the file, the driver leaves the tape positioned at the end of the
file.

You can test that theory by trying "dd if=/dev/nst0 bs=10240 count=1"
after rewinding the tape.