Disappearing DVD drive--not an upper or lower filter issue

OK, I am trying to help a couple guys out both suffering from this same
issue on Vista installations.

On both computers the DVD drives were working fine one day and then
disappeared the next; on one computer. One computer has sp1; the other is a
dual boot with out sp1.

There are no upper or lower filters listed in
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\Class\{4D36E965-E325-11CE-BFC1-08002BE10318}

The following keys do not exist
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Cdr4_2K
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Cdralw2k
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Cdudf
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\UdfReadr

They have tried uninstalling the secondary controllers that both drives are
connected to and rebooting--the secondary controllers are reinstalled as
expected, but the drives do not return.

The first drive, the one with sp1 being installed, is a TSST corp cd dvdw
sn-s082n. This drive worked flawlessly for months and suddenly disappeared.
There is no dvd (optical drive) listed in explorer, device manager, or disk
management. No relevant errors in event viewer. The drive is recognized at
bootup by bios, you can boot with a live linux cd with no problems. It was
used to install Vista. It's worth noting this drive is NOT on the HCL list,
though similar models are (s082d, for example)

The second drive is Optiarc 7530A/C. This drive NEVER worked in Vista. There
is no dvd (optical drive) listed in explorer, device manager, or disk
management. No relevant errors in event viewer. The drive is recognized at
bootup by bios, you can boot with a live linux cd with no problems. It is on
a dual boot machine and works flawlessly with Windows XP. It ALSO was used
to install Vista. However, it's worth noting this drive is NOT on the HCL
list, though similar models are (s082d, for example).

The first case is irritating since it worked flawlessly and then one day
stopped. I also know several people with this DVD drive and it works fine on
identical laptops. One thing worth noting on this laptop is that it had
difficulty with an sp1 install. During the sp1 install, the computer froze
and did not complete. The user rolled back the system and the DVD was
working still. He then reinstalled sp1 without incident. He noticed several
days later the DVD disappeared--he cannot say with certainty if he used the
DVD after the sp1 install or not. I cannot help but think damaged atapi
layer.

My inclination on the second one is to chalk it up to not being on the HCL,
but it bothers me that you could install Vista with it, and it works
everywhere else.

Thoughts?