It appears that this was not a dead battery either. It looks like after something caused the machine to boot to OS9, then slight differences in the file properties caused the Quickbooks datafile to be unreadable as well as the date/time. Whatever made the machine startup on OS9 was static for weeks until a crash, after which it booted on OSX (10.2 Jaguar) where everything is back to normal. Even the date/time is set accurately (which I had never manually reset on OS9).
I got curious and dug into your issue. You should read this:
https://www.macworld.com/article/1002820/QuickBooksPro5.htmlIt points out how QB5/Mac is is crap version with lots of problems. it was evidently not much more of a
carbonized version of QB4 - carbonized so it would run on either OS9 or OSX. It's not surprising that despite that alleged ability, it evidently can't recognize its own files on one OS or another. Couple that with the fact that 10.2 is hardly a perfected OS itself, and anything can (and apparently does) happen.
You also almost certainly do have a dead or at least a very weak battery. The startup system selection on dual-boot machines is one of the parameters held in PRAM, and PRAM requires a working battery to hold data reliably. The fact that the computer reset its own time/date is a feature of OSX you evidently aren't aware of.
Any ideas on what really happened? Or how to prevent this in the future. Can I delete OS9 from the machine entirely so it can't ever boot there? Or some better idea? (Since I won't be using the machine again until tax season next year, I may forget again what the correct OS version is. Although going through this this year will probably make me remember. This machine will stay on OSX 10.2 as long as it keeps working.)
As explained above, what happened is you have an old computer with old OS's and an old battery incapable of maintaining the settings for a whole year. To answer your questions:
1. You can absolutely delete OS9 entirely by simply locating the OS9 System Folder and deleting it. This will cause you to be dependent on 10.2 so you better make sure that everything you need works in OSX first.
2. The best way to prevent this in the future is to not leave your tax returns entirely dependent on an ancient piece of crappy software running on an old operating system (that I wonder if you have any original discs of) sitting in a closet depending on an old battery……do I really need to continue?
Some observations…
Tax returns are a pain in the ass.
Having tax software is supposed to lessen that pain.
Computers and applications and data need to be maintained - at least minimally if you want it all to work when you're staring at April 15th.
Almost nothing will cause more
unnecessary grief in your life that f**ing up your taxes.
Quickbooks 5 is was a lousy app to begin with and is now completely obsolete.
At least, get a new PRAM battery, backup your apps and data and consider updating your tools a little. You never said exactly which old Mac you're using, but at least updating it's OS to something newer than 10.2 would be a prudent move.