Hi!

First of all--if you get the data, then by all means, get it off that drive.
I wouldn't be too surprised to hear of a an ST-251 still working, as the
design was cheap and simple.

Unfortunately, these drives used a stepper motor to drive the heads around
the platters. This setup is touchy and subject to drifting out of
calibration over the years due to temperature shifts and drive movement.
When this happens, the stepper can't be positioned to find the data where
the drive and controller think it should be. You then get an error.

Back in "the old days" the suggested procedure was to keep backups, make
regular backups and make a backup prior to moving the drive in any way
whatsoever. It was also suggested to periodically run a low level format on
the drive to "realign" everything, after making a backup and hopefully
before the drive became unable to retrieve data.

As far as a solution goes, I'd highly recommend SpinRite from Gibson
Research Corporation. I never have run it on an ST-251 drive, but it
certainly existed at the time those were popular. I've seen drives that it
could and couldn't save...but the overwhelming majority were anywhere from
much better to completely salvageable. SpinRite costs around $100 USD and
comes with a 30 day money back guarantee if it doesn't work. I have no
affiliation with GRC other than as a satisfied customer of theirs.

William