I'm going to make an educated guess here, in the sense that I haven't proved this particular case.
A common question is "I put my system drive in a new machine, will it boot?" and the common answer is "probably not." The installation configures certain specific drivers to load early in the process to handle the motherboard, drive controllers, and other hardware. If that is not the hardware that is found, the process fails.
So booting from a USB copy of the OS is booting into radically different hardware, will get the wrong drivers, and will most likely fail for the same reasons as sticking the old drive into a new machine and trying to boot.
That said, if this is not your only copy of the drive, you could try my favorite solution for the different-hardware boot issue: Boot to the distribution DVD and do a Repair installation. If this is your only copy, for heaven's sake don't take the risk. Unless this is purely curiosity, pull the external drive out of its case, install it internally, and then boot.