USB flash drives are slow
you might have not 'ejected safely' and waited for the OK to do it Message.
(the Write buffers can take minutes to empty into the Flash drive. Longer if Anti-Virus is scanning
those files.) NB I copied a 3G library to a flash drive: It took HOURS.
IMHO get a 2.5in external USB Hard disk for that big (and it will have lots of spare room
) and be speedy