Any corrupted files on that external drive? Explorer will often baulk on corrupted files, especially corrupted video and image files if the view option is set to thumbnails or icons.
Another possibility, a virus scanner. Is it scanning the external?
I have a simpletech usb external hard drive that freezes up my operating system windowx xp home vr. 2002 with service pack 3 (delay write error message), when I transfer files from drive to pc. Is it game over for the my hard drive?