Vista - Vista is not assigning driver letters to external drives, claiming "Unknown Device"

My Vista Ultimate laptop stopped assigning driver letters to new external
drives, whether external hard drives or thumb drives. Old thumb drives I
used in the past are still recognized. This problem is happening on my new
Western Digital 1 tb external drive, whether using USB 2 or Firewire 400.
It is also happening on my new SanDisk 8gb U3 thumb drive and a new PNY 1gb
U3 thumb drive.
When inserted, Vista starts loading driver drivers for the device, but then
claims the drive is an "Unknown Device". For the external hard drive, the
driver software installation finds the WD My Book USB Device, USB Human
Interface Device, USB Mass Storage Device, USB Composite Device, and then it
fails to find "Unidentified Device". Device Manger shows the WD My Book USB
Device under disk drives, and also an Unknown device under Other devices.
Disk Management shows the drive, but without a drive letter. My computer
shows nothing for this drive.
All of these drives work fine on my older XP Pro laptop.
Please tell me why a drive letter is not being assigned to this "Unknown
Device" and how do I correct this problem.

- Michael Faklis <Michael_Faklis@xxxxxx>