> Try using "mt eew 1" before starting the backup.

OK, tried that - still no luck... :-(

The script "gendata" just does repeated dd's of a 4GB raw partition to the tape
until it gets an error.  As you can see from the following it got an error
after writing around 2GB in approx 11 minutes...

lido# /tmp/gendata
i = 0
dd: /dev/nrst0: Input/output error
4413440+0 records in
2154+0 records out
2258632704 bytes transferred in 667.468 secs (3383881 bytes/sec)
Jun 27 20:44:56 lido /netbsd: st0(ahc0:0:6:0): Check Condition on CDB: 0x0a 00
01 00 00 00
Jun 27 20:44:56 lido /netbsd:     SENSE KEY:  Media Error
Jun 27 20:44:56 lido /netbsd:    INFO FIELD:  65208320
Jun 27 20:44:56 lido /netbsd:  COMMAND INFO:  33478 (0x82c6)
Jun 27 20:44:56 lido /netbsd:      ASC/ASCQ:  Write Error

Any other suggestions for what might be causing this or how I can debug
it further?

Thanks,

Duncan