Follow this microsoft KB article and change the drive
letter using disk management tool.Microsoft Knowledge Base
Article - 307844.
if this doesn't work change through the registry.
Run the Registry Editor (REGEDIT.EXE).
Open one of the following branches, depending on the type
of device you wish to configure (your system may vary):
For all SCSI devices, and most non-SCSI CD-ROM drives,
open HKEY_LOCAL_MACHINE\ Enum\ SCSI.
For IDE hard disks, open HKEY_LOCAL_MACHINE\ Enum\ ESDI.
For standard floppy drives, open HKEY_LOCAL_MACHINE\ Enum\
FLOP.
Expand the branch of the SCSI device you wish to
configure, and click on the key under that device (if you
have two of the same device, there will be two keys here).
Double-click on the string value called
UserDriveLetterAssignment (create it if it's not there by
selecting New and then String Value from the Edit menu).
In the box that appears, type the desired drive letter
twice, in all caps (example: type NN to configure this
drive to use N

.
Next, double-click on the string value called
CurrentDriveLetterAssignment.
In the box that appears, type the desired drive letter
once, in all caps - if this device is partitioned into
more than one logical drive, include all drive letters
(example: type CEFG to configure this drive to use C:, E:,
F:, and G

.
Close the registry editor when finished, and restart your
computer immediately for this change to take effect.