Are you using a PC with Windows 7? When you first plug in the X, did the PC install a driver for the X (as though it were a flash drive - which is should be if the USB Mass Storage Mode is selected in the pull-down notification panel).
Also, did you install that (AWFUL) Moto desktop program that always wants to launch when you plug in the X as a USB/flash drive. If you did install it, UNinstall it from your PC and hit cancel any time it shows up on the screen thereafter. PITA, but that program is totally USELESS in my opinion, and only causes problems.
One other thing, your X can't be locked when you hook up the USB. If it is, the PC won't "see" it until you unlock it.