Hi Rob,
It is possible Ghost may not be able to recognize the drive under some conditions, usually it is the combination of Computer and USB Device.
Your USB external drive is formatted in NTFS or FAT32?
If it is FAT32, would you get a drive letter assigned when you boot to DOS?
If it is NTFS, it would show in the menu with the notation Drive.partition (For example 2.1) -So when you go to drive menu, you only get something line 1.1 (for the hard disk partition)?
Other option is using Windows PE and Ghost32 - Windows PE has better support than DOS for devices. Did you consider that?
Krish