aperiodic scsi tape problems: suggestions?

[Bill Tangren <bjt aa usno navy mil>]

I sometimes have problems with my scsi tapes. Today, with I try to do a 
retension on a tape, I get this error:

# mt -f /dev/st0 retension
/dev/st0: Input/output error
#

To say that this error message is not helpful is an understatement. I 
tried cleaning the drive: no luck. The drive is a:

Host: scsi0 Channel: 00 Id: 00 Lun: 00
  Vendor: Seagate  Model: STT20000A        Rev: 8.43
  Type:   Sequential-Access                ANSI SCSI revision: 02

This is taken from the /proc/scsi/scsi file.

Any suggestions on how I can debug the problem?

Bill Tangren