[...]

O.K., so you know ttape drives need regular cleaning. Hmmm.
Still looks like a hardware issue to me. First of all, don't
panic ;-)

There is one other failure mode I have observed in tape drives:
Some use a optical sensor to detect punched-out holes in the tape.
If there is dust in its path, it may fail. A cleaning cartridge does
not help for that, compressed air in the right place does. However
I do not know whether this can happen with the on-stream system. With
QIC-80 it was fairly common.

One other consideration: If OnStream is not in the market anymore,
that massively decreases the worth of your backup-system, since
what do you do if your drive breaks permenently? (Asuming it can
be repaired this time.)

Of course you could pay a commercial data recovery outfit to get at
your data when you need it, but changing to a newer system may be
cheaper and simpler. If you dos so, the consensus here seems to be
that for smaller-scale backups external HDDs or HDDs in swappable
bays are the way to go (some people here also advise CD-R(w) and
DVD+/-R(w), but these are not reliable enough for data with any
real worth) or alternatively 3.5" MOD or DVD-RAM for smaller backups.
The problem with tape drives is that they have not kept up with the
capacity increases of HDDs. They are still cost-effective in
tape-libraries with hundresd of tapes but not for smaller set-ups.

Arno