If this is a hard drive and not some solid-state storage device, overwriting the entire disk with plain ordinary zeroes will prevent anyone except NSA men in cleanroom spacesuits from reading the previous data. If your data thieves are that motivated, all bets are off anyway.

I'd make your patterns prime number lengths, perhaps 5, 7, and 11 characters, to give maximum annoyance to the cleanroom spacesuit people since this will make a very long-interval thing for them to have to predict and compensate for.