Zeroing will leave the drive "usable" in the sense that it will be a formatted blank volume ready for you to store things on. It won't have an operating system or any useable or recoverable information left. (As an aside, if you're
really paranoid about info being recovered from your wiped drive, I've heard that some folks over-wipe three times -- I suppose that's the IT equivalent of "where's Jimmy Hoffa?").
When you say "repair", zeroing will fix software issues, but only those that were caused by mis-written data blocks -- plus, if you don't have an uncorrupted version of whatever you want to fix, you may be out of luck. What zeroing won't do is fix portions of the disk that have gone bad. That's an issue with the drive surface (caused by a bit of oxidation, a little head contact, a bad jolt, some dust, etc.) where info can't be stored/retrieved.
Bad disk areas can be benign -- just a bad block. However, they can be a symptom of something that will eventually cause more damage. Use a utility that checks for bad blocks. If you seem to have an increasing number over time, consider a new drive. Installation into a G4 is a do-it-youself-er, as long as you're not that kid pushing on the pull door to the School for the Gifted...
