In case anyone else is interested in cheap backup tape drives with expensive
cartridges :-), I finally resolved the problem I had with this tape drive.
After getting the drive's SCSI manual from Conner, er, Seagate, I discovered
that you can't use a logical blocksize greater than 26KB (the tape uses a
512B fixed blocksize but will emulate different blocksizes).  I've long been
in the habit of using the tar program with gigantic buffer sizes, like 200
blocks ("streaming?  what's that?"), which it turns out confuses the tape
drive no end (but only when it tries to *read* the resulting curdled tape).
[The manual doesn't actually document this limitation, but the description
of the tape format offered the clue to what the problem might be.]

With reasonable (i.e. default) tar blocksizes, or with the driver set for
fixed blocksizes, the drive works quite well.  You can get internal-mount
CTMS-3200 drives for $179 from Corporate Systems Center (until they run out),
which is not at all a bad price even with cartridges that run $28 or so (for
2GB), unless you need a couple of dozen tapes for your backup plans.
There is a new version of the mechanism, the TapeStor 8000 I think, which
also accepts TR4 Travan cartridges; this unit is cheaper than (new) DAT 
drives, but the cartridges are markedly more expensive that DAT cartridges
(though the Travan format will probably be popular enough to drive the
price down, unlike the QIC-wide 3080 format).  I'd assume the TR4 format
has the same undocumented block-size limitation.