Excellent question! I am thinking about using USB hard drive to store images but my plan is a bit different from what you are doing.
At the moment I have a USB key that I have created for multibooting. I attaching the manual how to do it. Besides of the different LINUX distros and WinPE one of the DOS floppy images is used to boot to PS-DOS and run Ghost. The floppy was created with Ghost Boot Wizard - the trick was to create a short / text file named ghost.exe and point out Wizard to it. The actual Ghost.exe won't fit to floppy so it is located directly on USB (along with all other Ghost stuff).
Then I used WinImage to convert real floppy to image (that is being loaded completely to RAM by SYSLINUX).
I am booting PC from USB key. Floppy Image is A: drive, the USB key is recognized as C: drive. Hard drive in PC is identified as drive number 2 (from Ghost point of view). So I am launching Ghost from this C: drive (which in fact is just USB). Everything works like a magic :)
Hope it helps.