I am puzzled by this question. Under what circumstances do you have QNX4 hard drives that you can essentially throw away, but are so hard up for storage that you want to re-use them? I doubt these are particularly large drives; QNX4 was dying out around 2002-2003, and most QNX systems didn’t need a lot of space, so I’m guessing these drives are no larger than 80GB, if that. They are also probably quite old, which means possibly closer to failure.
Storage is ludicrously cheap these days. I have a half-dozen drives sitting on a shelf at home collecting dust. I just bought some 4GB SD cards and USB sticks for less than 10 bucks. A new 160GB hard drive from Newegg is $35, ($42 shipping).
While fdisk is the technical answer to your question, a more prudent route is probably to buy (or get someone to give you) a drive in a known-good condition.
Sage Automation, Inc.