When installed on a Windows server or stand-alone PC, the APPX license is tied to the serial number of the C-drive of the machine on which APPX is installed. That is, even if you are installing APPX on the D-drive (or any other drive), it is still the C-drive serial number that is required.
To determine the serial number, exit to a DOS (command) prompt, and type the command "dir c:\ /p" (without the quotation marks, of course). This will display the first page of a directory listing for the root directory of the C-drive. The second line displayed will contain the serial number, like this:
Volume Serial Number is 1A2B-C3D4
That is the number that APPX requires, but it will be reformatted when entered on your license and when displayed by APPX. APPX makes three alterations to the serial number: