I said, with drwilczur's method (changing the com.apple.boot.plist file) it worked for me but I had to restart the computer every few hours to keep it happening. I'm starting to wonder if that might not be some freak of my software configurations.
Has anyone else tried the fix and had it working permanantly?
INHIBITION FROM UPDATING
Neilarm's problem has been around for a long time now. A friend of mine still uses Windows XP. The rest of this post is about it.
I know of one way to deal with it but it's paid software (SuperDuper!: $28). You also need at least one external drive for a special kind of backup.
A SANDBOX BACKUP
SuperDuper! is primarily backup software. For a very basic backup function we can use it for free. For our purposes here, though, we can use one of its advanced functions, sandboxing. We create a special kind of backup called a "sandbox" on an external drive and then use this as our startup drive while we decide whether to upgrade or not.
The sandbox has a copy of all the system files but not our user folders. With it as our startup drive, all system changes (including changes to apps) affect only the sandbox, not our internal drive. Other changes — documents, user preferences, anything that lives in our user folder — are saved on our internal drive as usual. The user experience, though, is no different from normal.
If we apply a change to the operating system it affects the sandbox. Then we automatically see the result as long as we continue to start up from the sandbox.
If we stop using the sandbox as our startup drive and start up the old way from our internal drive, we revert to the old operating system (including the applications folder) untouched but we still have all changes to our user documents etc.
MY EXPERIENCE WITH A SANDBOX
I've used this and it works well. We get to try a system update and just trash it if it's no good (erase the sandbox). There are some things I'd suggest with it, though, according to my understanding:
1) Just use the sandbox as long as necessary. You could go on using it forever but the computer will run faster if the internal drive is the startup drive.
2) Be careful to keep the sandbox connected as best you can while using it. If it gets disconnected the computer is sure to crash. The more often that happens, the more the chance of software going bung.
3) Your old Applications folder stays as is on your internal drive, untouched, along with the rest. Changes you make to apps like updates are applied to the sandbox. When you stop using the sandbox you'll have to apply those changes again if you want them.
4) This kind of sandox is designed for people who have their computers set up the no-brain way: all their user stuff is in the usual places — user folders (home folders). At least that was the way it worked when I last used it a couple of years ago. However, people who have rebelled and put stuff of their own in unusual places — outside user folders — have to modify the sandbox manually to allow for this. I had to do this for a friend but it wasn't hard after all.
I've also used it for a system upgrade (not just an update) and the results were fine there too. In this case, though, there is a special thing to be wary of:
5) It's extra safety to have a backup besides the sandbox, a backup of the ordinary kind. Time Machine would do. Buy another drive for this if you don't already have one.
The reason is that after a system upgrade some stuff in your user folder may not work any more as is — things like your mail or photos. When you try to use these things, the newly upgraded operating system will ask you to let it change them in order to work a new way. If you just go ahead and click OK, they will work, but not when you stop using the sandbox. The old system on your internal drive won't understand the changes. If this happens, the only copy you will have of this old stuff without the changes will be in the Time Machine backup.
As far as I know, the developer of SuperDuper! has not built any extra feature into sandboxing to cope with this, but I may be wrong. However, if you just have the extra Time Machine backup, you can take it all to Apple's genius bar and dump it on them to sort out over a few days. That's what they're for, right?
AN EXTRA SAFETY MEASURE — A DUPLICATE BACKUP
For those of us who are well enough off to afford extra drives, yet another backup can be useful here too — a duplicate or "clone" backup of the traditional kind (not a sandbox). This gives you one more option for solving a problem. The software I know of that do this well are SuperDuper! and Carbon Copy Cloner.
It is ideal for the situation where you just want everything back the way it was in a quick, no-brain do-it-yourself way. You make the duplicate at such and such a time. When you want to trash any and all changes on the computer since that time you just boot from the duplicate and do a restore — a backup from it in reverse, to your internal drive.
If you use a duplicate for backup in this way, be aware that it's an extra thing for convenience — not a substitute for a Time Machine backup. They're different kinds and help you in different ways. For ordinary computer use if you only want to have one backup, it should be a Time Machine one.