Does the drive need to show more free space than a synthetic full would take.....even though it would not actually need the space?
Not sure if I got it right, but I'll try to answer:
FastClone should work just fine with USB HDD. If it worked then if you add up all the sizes of all backup files you'll see that the number is smaller than Used Space in drive's properties. Can you see "[fast clone]" in the session statistics?