The simplest means of getting rid of your files is to delete them, then delete all of the temporary files in the temp folder (on Windows OS), and clear the internet cache. Make sure to empty the recycle bin. Then, defrag the drive. I doubt if there will be any recoverable data after that.
That wouldn't do it-there are plenty of tools that would be able to recover the data on those drives. Understand this-deleting a file only deletes the first character in the file name, but the entire contents of that file remain until they are overwritten. Your best bet would be to format the drive, then run a program off of a separate boot disk that will write a single character on every spot on the disk.