>>I want to know for example on which of the two (for example) hard disks resides drive letter 'E:'. I wish to determine this programmatically, from my own program.
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>There is a correlation between the drive letter and device in the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry key data. However, I haven't figured out what that correlation is, nor can I find an API call to get this information. Disk Manager seems to be able to figure this out, though, so there's got to be a way to do it.

Yep, but how to filter the junk under this registry key... (I mean there is left information about drives that I had in the past, but have now disconnected them)
Also - this key is not found in Windows NT 4.0 (both Server and Workstation) SP6a and under Windows 9x/ME