Enabling write cache for a Maxtor DM9 160 GB PATA on a GigaRAID(ITE) or Sil3112?

Hello!

I recently got a Gigabyte 8KNXP board and am so far very happy with
it. There is just one problem, my "old" Maxtor 160 GB PATA drive (8 MB
cache) apparently does not have write cache enabled when I connect it
to the onboard "GigaRAID" RAID controller (a ITE chip, works with a
single drive in RAID mode but can be configured to ATA too). The read
transfer rate is as expected but the write transfer rate is horrible
(3-7 MB/s...). First I thought that the ITE controller is just sh*t
but then I tried a Seagate Barracuda and it showed normal write
transfer rates.
Then I tried putting the drive on the onboard Silicon Image 3112 SATA
controller via a PATA/SATA adapter on the Maxtor HD (the adapter is
also from Silicon Image). Here again the read transfer rate was ok,
but the write transfer rate the same as on the GigaRAID chip -
horrible.
At this point I figured out that the disabled write cache is the
problem as I could see this in the thingy the Silicon Image drivers
put into the control panel.
Then I tried the drive on the Intel onboard PATA controller. Well on
this one the write cache is enabled and the write transfer rate is
normal (up to 50 MB/s, usually 30 MB/s).
When connected to the Intel I can activate write cache at the drive
properties in device manager. When I connect the drive to the Silicon
Image or the GigaRAID there is no checkbox there. I also tried
checking the tools that are installed with the SiIm/GR drivers but
there seems to be nothing there to enable the write cache, just the
SiIm shows me that write cache is disabled (but I don't see a way I
can turn it on).

Now you could say that I should just keep the drive connected to the
Intel controller, but I have 4 optical drives and the GigaRAID does
not support optical drives.(I do have one of them connected to the
Silicon Image via the adapter right now but the HD is still on the
same cable as another optical drive on the Intel controller and I'd
prefer if the HD was not connected to the Intel controller together
with a optical drive - of course I could buy another adapter
though...).

So is there any way to activate write caching (maybe some registry
setting or something) for the Maxtor HD on the Silicon Image or the
GigaRAID controller?

btw the system boots from 2 SATA HDs in RAID0 mode from the Intel SATA
ports so there is no way the PATA HD can go there with the adapter.

one more thing I forgot to mention: my 160 GB Maxtor DM9 has a Y6
serial number, so it's an older one with 3 platters (not 2 80 GB
platters like the newer ones probably have).