Hi, I don't know if it would work, but I advice against using a momentus XT for an external drive. The momentus XT is a good boot drive but you don't benefit from the speed the SSD part in it gives you when you use it for data storage.
Buy a less power hungry and expensive 2.5inch hd or directly a WD portable(or any other portable 2.5inch HD) which will work with usb 2.0 and also usb3.0 (for a little extra price) out of the box.
I just googled what you said and you're right, seems the XT only benefits from the SSD as a boosted cache so it'll only help in read speeds, not really very helpful for an external drive...
Thanks for your advice