disknum (ULONG) - input
This parameter can be any value from 0 through 26. If this parameter is zero, information about the disk in the current drive is retrieved. Otherwise, 1 specifies drive A, 2 specifies drive B, and so on.
When a logical drive is specified, the media in the drive is examined (for a local drive only), and the request is passed to the file system driver (FSD) responsible for managing that media, or to the FSD that is attached to the drive.