How confidential is the data on the disks? Reformatting will certainly remove the data to a level sufficient to conceal for Mr Average user, but for an IT-savvy user, they can still get at the data.
You need to use a third party utility that does through and overwrites every track with 1s and 0s (or other random data) several times over to fully kill the data to a high level of security. Even then, a laboratory can retrieve the data but at great expense and time.
Search Google for Disk Eraser. There's many available.
Regards: tf1