If you have compressed a whole drive and then add a hard drive or CD-ROM drive to your system, then not all drive letters might show up in the Explorer.
When you compress an entire drive, DriveSpace gives the host drive's original letter to the compressed drive and assigns the host drive a new letter, several letters past the last letter actually in use. For example, if you compress all of drive D: and the last actual drive is E:, the host drive might get assigned the letter I:. When Windows steps through the list of drives, it has to ship the letters between E: and I:. It appears that each bit in the NoDrives Registry key represents a drive letter and a set bit means that the drive should be ignored. For instance, the value 192 expressed in binary format is 11000000. Considering drive A: as rightmost and working toward the left, you'll find that the two bits that are set to 1 represent drives G: and H: and those drive letters will be suppressed. The NoDrives key is undocumented, so its use might change in future releases of Windows.
You can restore the display of missing drives by editing the Registry directly. Editing the Registry should be done only by experienced users and with extreme care and the Registry should always be exported or backed up before any changes are made.
1. Start the REGEDIT.EXE program.
2. Go to the key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Policies\Explorer
3. Double click on the key NoDrives and enter a value of 0 to display all drive letters or enter a binary value where each suppressed drive is represented by a 1 and each displayed drive by a zero, starting with drive A: from the right.