To wipe a hard drive using zeros:
dd if=/dev/zero | pv -s 250000000000 | dd of=/dev/sdX bs=10M
To wipe a hard drive with random data (more secure):
dd if=/dev/urandom | pv -s 250000000000 | dd of=/dev/sdX bs=10M
For the "pv" progress bar, do "apt-get install pv" or similar. I highly recommend using "pv" so you have some idea of when the dd job is going to finish.