Yes your idea about creating a very large file is basically correct, but because the file system is in use and the operating system is running, the amount of free disk space will vary slightly all the time, meaning the data you wanted to get overwritten may not get overwritten. You can do this yourself using the 'dd' command (there's no timer or anything so settle in as it may take a while), although I think this is pretty much all that Disk Utility does anyway:
dd if=/dev/zero of=/huge.file bs=512 && rm /huge.file [/CODE] That will create a file in the / directory and call it 'huge.file', then fill it with nothing but zeroes from '/dev/zero' until the disk runs out of space (using a block size of 512). When the dd operation is completed and the drive is full, the next part of the command will delete the huge file to free the space again.