DLT8000 too slow on desktop PC

We have a DLT8000 (HP SureStore DLT 80) which is connected to a desktop
PC with a normal ATA hard disks. The SCSI controller is Adaptec 29160
U160 and the tape drive is the only device on it. The cable is the
default that came with the drive (LVD/SE around 1 meter long).

Backing up from the hard disk to the tape using Windows XP backup is
too slow. I read about source hard disk speed having to be high enough
to keep up with the buffer of the tape drive. Some suggest 12MB/sec for
the hard disk. The benchmark on our hard disk is 5MB/sec.

HP Tape and Library Tools Read/Write test reports that writing rate was
0.4MB/sec

What can we change to get the full performance of the device? Will SATA
hard disks help?

More info that might help you out there to tell me what could be wrong:
LTT tests show:

Tape drive perf:
116 MB written in 300 seconds at 395 KB/sec

Backup pre-test:
131 MB read (6 files) in 60 seconds with rate 2 MB/sec
TOTAL: 131 MB read (6 files) in 60 seconds with rate 2 MB/sec

The disk may be slow because it is fragmented, i.e. wastes time seeking
instead of transfering data. You can watch the transfer rate with
PERMON (Physical Disk, Disk Bytes/sec.)
Defragmenting the disk before the backup may help, but if you're
backing up directories with large numbers of small files you may be
stuck. Turn off the Indexer service and any virus checkers to avoid any
competing disk accesses.
A fast disk (15 krpm) may help. The interface doesn't have anything to
do with it.

Ralf-Peter