Reference: Map Network Drive
This drop-down list displays those drive letters that are available and those that are already connected to a remote resource. Local drive letters are not displayed.
The list will vary depending on the platform on which Beyond FTP is running. On Windows NT and Windows 2000, drive letters are a global resource. Beyond FTP will attempt to determine which drives are currently mapped by any user on the system, including Beyond FTP, the console user, and other system resources.
On Windows XP and Windows Terminal Services client sessions, each user has a full set of drive letters. Under these conditions, only the existing Beyond FTP drive mappings are displayed.