Drive dissapears in Windows, Error 9 in System Log

"The following Intel article seems to address your problem:
Intel Rapid Storage Technology : Possible issues with Windows Vista* and Intel® RAID

Some excerpts from the text:

Description

The following issues may occur on
Windows Vista* with a supported Intel®
RAID controller hub:


Timeout errors reported by iastor or iastor(v) in Microsoft Event Viewer


Cause

Various desktop Serial ATA devices
such as hard drives and CD/DVD drives
have been reported that do not comply
with the Serial ATA Link Power
Management (LPM) device specification
and display erratic behavior when LPM
is enabled.

Solution

Install Intel Matrix Storage Manager
version 7.5 or later. These versions
workaround this issue by disabling
Link Power Management (LPM).

If you are experiencing installation
issues, you can also use one of the
following additional workarounds:


If available, install updated firmware for the device.
Attach the device(s) to ports 3-5 on the Intel® 82801HR/HH/HO I/O
controller hub (ICH8R) or port 3 on
the Intel® 82801GR/GH I/O controller
hub (ICH7R)
Switch to IDE mode in the system BIOS.
Note : This change will disable RAID
functionality. Contact your BIOS
manufacturer for instructions on how
to switch to IDE mode."

"put a small page file on the drive that is getting disconnected--it could be an orphan power management issue and a page file will keep the drive spun up

edit:
actually, re-reading your OP, if the drive is showing up in explorer but you can't access some of the data, it sounds like the drive itself is failing. can you reproduce the issue if you attach a different drive to the same port?

also try replacing the SATA cable first since it's cheaper than a new drive"