Originally Posted by rayzz
thanks, worked perfectly! I should've guessed something like this, but now I know.
*nod* You'll want to find the proper unmount methodology for Ubuntu (sorry, I'm a Fedora nut) that does this more gracefully; on my systems the device mounts right on the desktop automagically, at which point I rightmouse on the icon an "Unmount Volume" for a clean removal. This issues a cache flush and runs the 'umount' command behind the scenes.
If nothing else, you should at last unmount the volume (which would auto-sync as per the above reply), something like 'umount /mnt/usbdisk' -- wherever that /dev/sda? device inserted itself onto your filesystem. See this reference I wrote here: Reference: mass storage + linux
There are some examples of bad output from just yanking the USB cable out, and good output when you properly unmount your USB device.