There's been an issue with network collisions with larger files causing kernel panics. Your issue *might* be related. The fix I got for the kernel panics was to go to the raspberry pi github and replace the kernel.img file in /boot with the one located here. Just click on the button that says "raw" save the file and then get it onto /boot on the r-pi SD card. I went ahead and made a .bak of the current kernel.img just for safety.
I have experienced the kernel panics - thus I moved my USB2 disk back onto the NAS. So thanks for the updated kernel image link. Much appreciated
Video playback does not cause kernel panics... the videos just end a couple of minutes too early. xbmc continues to respond and I can select other media to play.
It might still be related, but without the kernel panic it does seem unlikely.