My computer will start out copying to a usb flash drive at 80+ MB/s, and progressively slow down until less than 5MB/s. Then at the end of the file copy, it will stall out for a while and finally close the save window. The drive is rated at 30+MB/s and certified usb 2.0. The port is also usb 2.0. What can cause the usb bus to slow down and act this way? Trying to copy files that are greater than 4GB onto NTFS filesystem.