"This device does not allow its write cache setting to be modified"

When trying to enable write caching in the HD disk properties of Device
Manager, the aforementioned (subject) line appears just below the tick box
to enable it. However, the following registry entry is set to "1":


HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\I DE\DiskWDC_WD1500ADFD-00NLR1___________________20.07P20\4&35cc0339&0&0.0 .0\Device
Parameters\Disk\UserWriteCacheSetting

The HD is set up as an AHCI volume on an Intel ICH7R controller (Intel's
Matrix Storage Console software installed) and the OS is Vista Business x86.
Is there something I'm missing, or is it because the drive is an AHCI
volume, as opposed to and IDE one, that I cannot enable it? Thanks.

--
RJ