How to quickly wipe a disk?

ATA Secure Erase is available on non-SSD drives, too. See https://askubuntu.com/questions/42266/what-is-the-recommended-way-to-empty-a-ssd on how to request the operation.

The fastest (and also the safest) is to encrypt the whole disk when it is new. Then, to erase, just erase the encryption key. Done in half a second, unfeasible to recover any data.

A really quick and easy option for magnetic disks is a degaussing bulk eraser. 20 seconds and your data is gone forever.

If you don't care about actually destroying the data on the disk, you can probably do something like dd if=/dev/urandom of=/dev/sdc bs=1M count=2 to fry the first couple of megabytes (which would include the MBR and partition table).

Be super-duper sure about which block device you point at as there are no taksey-backsies on this command