Recently MacFixIt reader Elliot Rosen wrote "i am going to purchase a new G5 and want to donate my G3/350 blue and white running Mac OS 9.1. What is the best way to erase-clean my hard drive of all my personal info?"
In Mac OS 9.x, the easiest way to erase your startup volume is to start from another valid startup disk, such as the Mac OS 9.x Installation CD. When you erase a disk, the Format pop-up menu gives you choices that include Mac OS Standard (HFS) and Mac OS Extended (HFS Plus). As explained by Knowledge Base article #50445, Mac OS Standard format is compatible with all versions of the Mac OS. But Mac OS Extended is compatible only with Mac OS 8.1 or later. If you format a hard disk with Mac OS Extended format, you can't use that disk with computers that use Mac OS 8 or earlier."
You can then simply re-apply a fresh installation of Mac OS 9.x, and ship the computer on its way.
If you want an added level of security, you may want to investigate a secure-delete utility, which will overwrite areas on a disk several times with random bytes in order to ensure that the next user cannot use a retrieval program to expose previously deleted files. Two that are compatible with Mac OS 9.x are:
UPDATE: A MacFixIt reader points out an article at rixstep.com, explaining that secure delete utilities provide a certain level of protection, but are by no means absolute in their destruction of files.
"It's hardware - and not software - that is used by the serious players to reveal secrets on your hard drives. Software stuff like EnCase is for kiddies - for cops who like to pretend they're J Edgar Hoover. The 'software' tools are easily defeated - it's the hardware tools you have to watch out for. They're merciless. They work by constructing a signal map of your entire hard drive, and then comparing this map, byte for byte, with what should be there. From the discrepancies they can backtrack several overwrites to find out what you had before you overwrote your files - several overwrites ago. Yes, it's that scary."
Other readers suggest using the Apple Drive Setup utility to write zeros to a drive. This is a long process, and like the above-mentioned applications cannot prevent fully against access to deleted files.