Hello,

>> By the way: I never EVER knew of a re-writable FS on tape, not even
>> with individual blocks, [...]
>
> DEC did it with the old DECtapes, on PDP-11s.  That was a very simple
> file system, of course.  Later, VAX/VMS at least pretended to have a
> re-writable file system you could use on 9-track tapes, but that may
> very well have been faked in software.

HP supported something like that on HP-IB tape drives ( like the 9144 
or so ) under early versions of HP/UX and (probably) HP Basic.

have fun
Michael