Sierra is now just around the corner, probably due on 20 September. If you’re intending to upgrade to it this side of Christmas, now is the time to start preparing. Chances are that your upgrade will complete seamlessly, and you’ll not suffer any significant glitches. The trouble is that this is unpredictable: even clean, modern Macs can suddenly take a serious dislike to this sort of major upgrade. If you haven’t made proper preparations, that can leave your Mac in trouble.
Check free disk space
This may seem obvious, but it is always worth ensuring that you have ample free disk space, or have a really good and determined bout of housekeeping to make it so. Aim for sufficient for Sierra, say, 10 GB, plus another 100+ GB to accommodate scratch files, etc. Anything over 500 GB is really comfortable.
If you can’t muster that, consider getting your internal drive upgraded before installing Sierra.
Clear out old cruft
If your Mac was upgraded to El Capitan, or migrated from a Mac running Yosemite or earlier, it is likely to contain old extensions, daemons, and other material which can cause problems if carried over into Sierra. In theory, Apple’s installer, Migration Manager, and other processes should check old components before they are moved over, but in practice we know that only the most serious of problems, such as kernel panics during startup, are prevented.
It is worth checking through /System/Library/Extensions to ensure that there are no old third-party kernel extensions left there; although SIP prevents new extensions from going there, old ones may have been migrated to it. There are several folders in the main /Library and your Home folder ~/Library which easily collect old material: check Extensions, Internet Plug-Ins, LaunchAgents, and LaunchDaemons in particular.
Your aim should be only to have current Apple components and services, and those third-party services which you need, and which are fully updated.
Update/upgrade to the latest OS X
All previous experience is that you are most likely to encounter problems when the difference between your current version of OS X and the one to which you’re upgrading is greatest. Going from 10.11.6 to 10.12.0 shouldn’t be as risky as going from 10.9.5 to 10.12.0. So the first step is to ensure you’re running El Capitan 10.11.6.
As we know, 10.11.6 isn’t as flawless as it should be, but you shouldn’t have to suffer it for too long. It also gives you a good chance to do the other recommended actions.
Update apps and the rest
Work systematically through all your major apps, on which you rely, and bring them all as up to date as possible. Soon after Sierra is released, most of the better vendors should carry information about compatibility with the new macOS, which may help.
This is most important for drivers and other software which supports peripheral devices: Sierra will come with updated printer and scanner drivers for a wide range of models, but some older products will inevitably lose support. If you have vital peripherals whose support is uncertain, keep a Mac running an existing version of OS X so that you can still use them.
Check your backups
This is one time that you must ensure that your Time Machine and/or other backups are in perfect order. In the event of disaster, such as having to revert to OS X 10.11.6, or losing any files during the upgrade, you will be relying on your backups to do that. So browse them using Time Machine, perform some tests such as restoring an old working document, and a folder or two. Check your logs for some recent backups and verify that there are no significant errors being reported.
Working with a belt and braces may be boring at times, but boring means fewer shocks and less downtime. If you have a suitable drive (an external USB 3 or Thunderbolt drive, for instance), make at least another copy of your Home folder. If you have the time and free space, why not use Carbon Copy Cloner to make a complete mirror of your startup drive before upgrading?
Keep the emergency exit clear
If you have more than one Mac, plan to upgrade one to Sierra first, then see how it pans out. When you’re happy that it does everything that you need, and none of your peripherals or apps have been orphaned by it, you can upgrade the rest of your fleet.
The only snag with this is that you will not be able to use some of Sierra’s new features across multiple Macs: if those are important to you, you may need to commit all more quickly. If you do that, bear in mind that you are giving up that emergency exit which might allow you to continue using that old scanner, or the old version of Adobe CS.
Keep an eye on blogs like this, and other sites, which bring timely information about Sierra, its problems, and incompatibilities. We’re here to share what we know and learn, and to help. Print magazines like MacFormat will also be publishing a lot of helpful material aimed at aiding you get the most out of Sierra. If you don’t subscribe to the print or electronic editions, this may be a good time to do so, as newsstand copies are likely to disappear quickly.
Sierra does require a reasonably recent Mac, of course. If yours is not officially supported, you may still be able to install Sierra on it, using the macOS Sierra Patcher Tool. I wish you success.