I do not recommend using attached USB disks to store media, because UpdateMedia frequently scans the disk. The nature of how USB is designed means that the CPU polls for any I/O to do, and thus greatly increases iowait, and therefore, can in extreme cases cause the system to experience long timeouts.
You can use the web admin's file sync option to copy the files, or you can connect to the \\dcerouter network share, and copy the files, that way.
-Thom