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SCSI tape issues

I'm running debian 3.0 woody on an SparcStation5 that I want to use for
backups of all my systems, but I'm stumped as to whether the second hand
tape drive I have is bad or I have forgotten something. My previous tape
experience is with a old Colorado unit via ftape; thus my confusion as I
believe I had crossed off the obvious, such as cleaning the heads & loading
the tape

from /var/log/messages:

esp0: IRQ 36 SCSI ID 7 Clk 40MHz CCF=8 TOut 167 NCR53C9XF(espfast)
ESP: Total of 1 ESP hosts found, 1 actually in use.
scsi0 : Sparc ESP100A-FAST
scsi : 1 host.
Vendor: SGI Model: IBM DNES-309170Y Rev: SAF0
Type: Direct-Access ANSI SCSI revision: 03
Detected scsi disk sda at scsi0, channel 0, id 3, lun 0
Vendor: HP Model: C1533A Rev: 9503
Type: Sequential-Access ANSI SCSI revision: 02
Vendor: TOSHIBA Model: XM-5401TASUN4XCD Rev: 1036
Type: CD-ROM ANSI SCSI revision: 02
Detected scsi CD-ROM sr0 at scsi0, channel 0, id 6, lun 0
scsi : detected 1 SCSI cdrom 1 SCSI disk total.

and issuing #insmod st:

Using /lib/modules/2.2.20/scsi/st.o
st: bufsize 32768, wrt 30720, max buffers 5, s/g segs 16.
Detected scsi tape st0 at scsi0, channel 0, id 4, lun 0

after googling for why I couldn't get tar to write to the tape, it seemed
the cure was stinit. So, I get a long output finished with :

Initialized 1 tape device.

mt seems to see the drive fine, by #mt -f /dev/st0 status:

SCSI 2 tape drive:
File number=0, block number=0, partition=0.
Tape block size 0 bytes. Density code 0x24 (DDS-2).
Soft error count since last status=0
General status bits on (41010000):
BOT ONLINE IM_REP_EN

However most other operations with mt either bail out immediatly with a
/dev/tape: Input/output error ; or they cause the tape drive to move back
and forth over the beginning of the tape for about a minute before winding
to the end of the tape, seemingly stopping for a minute, then completly
rewinding. Usually this produces what seems to be a controller error in
messages. This is accompanied by a "tape is write-protected" error
message, even though the tape isn't ro. If I try tar anyways with a test
directory & #tar -cvf /dev/nst0 tape/ I get the same movement as before.

It seems the proper settings for the dips on the drive's underside are set
right (ie. to default), & I have an otherwise working scsi chain. I'd hate
to think I've missing something so obvious, but this really has me in
stumped and newbie mode.