I think u r USB have some fragments....;First check for that by
it is better if u post the output of that then it is very easy to identify the problem...
Then mount the USB drive by doing
mount /path/of/source /path/of/destination
If u have the fragnments then perform defragmentation by doing
then it shows no.of nodes in that thumbdrive and no.of available nodes.The helth of the disk is estimated by the no.of nodes.If the nodes are equal to the available nodes then there is no fragments.If not there is a chance of fragmentation.
Then it asks the permission to defragment the drive.u just press 'y'.mail me if u have any problems regarding this session.