[DanBC]

This looks like a useful guide.

> If you're even a little paranoid, you should start by overwriting the disk with random data. We'll assume your hard disk is sd0—you can use dmesg to check. The c suffix is OpenBSD's way of specifying the entire disk.

   dd if=/dev/urandom of=/dev/rsd0c bs=1m

Can I check: why would you do this rather than using ATA SECURE ERASE command?

Having a blob of random data on my drive would mean crossing international borders is potentially unpleasant.