I just put together a box with:

  mpt0 at pci2 dev 3 function 0: LSI Logic 53c1030 Ultra320 SCSI

and two

  sd0 at scsibus1 target 0 lun 0: <FUJITSU, MAS3735NP, 5B08> disk fixed
  sd0: 70007 MB, 27094 cyl, 8 head, 661 sec, 512 bytes/sect x 143374650 sectors
  sd0: sync (6.25ns offset 127), 16-bit (320.000MB/s) transfers, tagged queueing

When I plugged the

  st0 at scsibus1 target 6 lun 0: <BNCHMARK, DLT1, 5032> tape removable

in, sd0 and sd1 disappeared. Tech support is claiming this is normal,
and that the tape has to be the only scsi device on the bus. I am
incredulous. What do you think?

Agreed the disks can cope with 320MB/s rather than 20MB/s transfers...

Trying the tape drive on its own, I get:

st0: sync (100.00ns offset 15), 16-bit (20.000MB/s) transfers
st0(mpt1:0:6:0): command timeout
mpt1: timeout on request index = 0xfe, seq = 0x000000a7
mpt1: Status 0x00000000, Mask 0x00000001, Doorbell 0x24000000
mpt1: request state: On Chip
st0: error 16 trying to rewind
st0: WARNING: cannot revert to default density

Is bad tape drive (communications) a possibility? or more to do with
the medium?

Cheers,

Patrick