RoadKil’s Disk Wipe is a very small (55Kb) portable utility to quickly wipe a full external hard drive, it will also wipe thumbdrives and memory cards. It is very easy to operate, after launching the program you select the external drive to wipe and if you want to blank it filling it with zeroes or fill it with random data, another setting lets you input the number of times the device must be overwritten.
This free data wiper doesn’t need administrator rights to run, you can take it with you wherever you go. Do not be fooled by its simplicity and lack of features, this program will make any data stored in a memory stick or external drive irrecoverable. I would use a three pass data wiping in case of an old drive skipping a sector, the second pass might as well go over it. But this would not happen for sure, when a drive has dead sectors the only sure way to destroy all data is physical destruction.