Is your USB hard drive USB-2 or USB-3 ?
Does your toughbook come with any USB drivers or USB chipset drivers that you need to add to WinPE in order to be able to use the USB port?
I've only just spotted this posting from 2 months ago, but I recall working with a Panasonic toughbook two or three years ago and there were no problems imaging to an external USB hard disk running WinPE.
Might be worth checking what version of Ghost you are using.