BackupExec runs as a service, this means that any drive letters mapped for a logged-in user basically "does not exist" from BackupExec's point of view.
Only local drives (as in "made available by a device driver") are visible to such services (By the way, this is also why you cannot share a mapped drive in Windows)
The solution is to specify the UNC path directly to the shared resource. Unfortunately, BackupExec is too braindead to understand that an UNC path may contain the character "." in an IP address or a FQDN, so you'll have to edit "C:\WINDOWS\system32\drivers\etc\hosts" or make sure the host name can be resolved via WINS or DNS. Further, if you are using NFS, note that Windows does not distinguish between the formats \\host\share and host:/share (this ofcourse means you can't mix SMB/CIFS and NFS on the same host)
