Originally Posted by rui-no-onna
From version 4.0 Ice Cream Sandwich (well, started with the tablet-only 3.0 Honeycomb, really), Android uses the Media Transfer Protocol (MTP) instead of regular USB Mass Storage Class (MSC). I don't have the Xperia U but on my Xperia Acro S, I have the option to use either MTP or MSC. In MSC mode, only the microSD card is accessible. Since the Xperia U doesn't have a microSD slot, it probably only works in MTP mode.
Odd...because when I coonect my Nexus 10 to my laptop, it reads it as a drive...and I move files around seamlessly....though come to think about it, the N10 is not recognised as a USB device...its just recognised, if you know what I mean.