It’s so tempting to join Apple’s public beta-testing programme and try macOS 10.15 Catalina out, isn’t it? Not only will it give you a better idea of what’s coming this autumn/fall, but maybe it will help you prepare for it now. And when you’re chatting with other Mac users, it’ll put you ahead of the pack.

In a word: don’t. Well, at least not unless you have thought this out more carefully.

I learned my lesson many years ago when a friend, the great Mac developer Patrick Buckland, was working on some disk security and encryption software for his publishers Cassady and Greene. Patrick developed the wonderful and highly-addictive game Crystal Quest, and I had taken for granted getting access to early releases.

At that time, I had but a single Mac II, on which I developed commercial software for the design and manufacture of sails and other products which were fabricated from panels cut from sheet materials using ultra-large format laser cutters. I installed this exciting beta release, and it not only encrypted my single internal 40 MB hard disk, but locked me out completely.

For a couple of hours, I was beside myself, until I managed to contact Patrick again. Thankfully there was a simple workaround, as it was still a beta, which I seem to recall involved a large paperclip and the keyboard. In a few seconds, my Mac II was back to normal, and I was able to get on with my own development.

This week, Apple released Catalina beta 3 to developers; I’m rather hoping that it won’t be put into the public beta programme, and that developers will get a fourth beta very soon. By all accounts, within very few hours of its release, some of its less endearing changes had been discovered. According to those reports, it can gobble up the contents of your iCloud Drive, and accessing any Help book logs you out of your user account the moment that you quit that app.

The previous beta seems to have had far fewer problems, but when they occurred they could brick some Macs altogether: this happened to a T2-equipped Mac mini owned by an extremely experienced developer. Something clearly went wrong with the T2 chip and its firmware, and the only way to recover the mini was to replace its logic board. That’s not the sort of thing that you want when you’re up against deadlines, or need access to that Mac for anything important or timely.

Although I hate to rain on anyone’s parade, or spoil your excitement at the new, my golden rules for beta-testing system software are:

1. Only ever install beta versions of macOS or any software which works at a system level on a separate Mac, which you don’t need for anything else, and could do without for a week or two if something serious goes wrong.
2. Avoid installing on a Mac which could readily get bricked, at present those with T2 chips, unless you’re prepared for them to go away for a new logic board and all that causes. And make sure that they’re covered by AppleCare, or disposable.
3. By all means dual-boot your testing system, but don’t expect its release version of macOS to work properly, and don’t rely on that version of macOS to make Time Machine backups, which may well break because of the beta macOS.
4. Installing the beta in a Virtual Machine can be much safer if it’s possible, but may not work at all, may behave differently, and will be dog slow. You still don’t really want to try this on a production machine, though.
5. If possible, set the beta system to use a different iCloud account, to isolate it from the account which shares your working documents and keychain. If you can’t do that, ensure that the contents of your iCloud storage are thoroughly backed up locally.
6. Remember that a beta is for trying out and testing. Never expect to get anything useful done with it.
7. Some betas are lemons. If you start sucking one and it’s clearly going to make trouble, avoid using it until the next beta is available. That will spare you a lot of pain and grief, and anxiety over what will hopefully be fixed next time around.

One important myth to bust is that it’s safer to join a beta-test programme later, as most of the bugs will be fixed by then. Developers like Apple can introduce serious bugs even in very late betas (sometimes in the final release), and it’s quite common for some quite radical features to be rolled out during the course of beta-testing. Just because it says that it’s beta 4 doesn’t mean that its risk is any less than the very first beta. If you want to reduce risk, follow the steps above, or wait until it’s released.

One thing that betas are excellent for is refreshing your practical skills at system first aid. If it’s a while since you last reset an SMC, or used remote Recovery mode, this may be a chance to remind yourself just what to do. If you support Macs for a living, look on it as being continuing professional development, a first aid refresher with real casualties. If you can’t face the blood and guts, then betas are not for you.

To paraphrase the Latin, caveat tester.