Possible Duplicate:
Securely erasing all data from a hard drive

In my personal collection are an increasing number of relatively new drives, only put on the shelf due to upgrades; in the past I have never sold hard drives with used machines for fear of having the encrypted password databases that have been stored on them compromised, but as their numbers increase I find myself more tempted to do so (due to the $$$ I know they're worth on the used market).

What tools then exist to make the recovery of data from said drives difficult to the extent that selling them could be justified? Another way of saying this would be: what tools/method exist for making the attempts at recovery of any data previously stored on a certain drive impractical? I assume that it is always possible to recover data from a drive that is in working order. I assume also there are some methods for preventing recovery of data due a program called dban, and one particular feature in macOSX that deals with permanently deleting data from a disk.