Can you look on the bottom of your X7 please, and let me know what the sticker says? There are a few different versions of the X7, hopefully I've got one of the type you have and we can check the path USB devices will take.
To double check it is working OK, open up the ES File Manager application (outside of XBMC) and expand the LOCAL menu on the left. Something like SDA1 should be displayed towards the bottom of that section, possibly with a little eject icon. Click the link (not eject!) to the device and make sure you can see the files. You can play them from here using MX Player if you like.
If they do appear, go back/up one directory at a time (until you reach the root / ) to find out where in your filesystem that shortcut took you. You should then be able to follow the same path (in reverse) within XBMC. If not, let me know the info on your sticker and I'll duplicate your set up.