When you have decided to upgrade to High Sierra, the next step is to prepare for the upgrade. This is the first time that macOS (and Mac OS X, and OS X before it) has undergone a file system upgrade, and the most fundamental change in Apple’s operating systems since it released the Mac OS X Public Beta seventeen years ago.
There are two major risks in the upgrade: first, that the conversion of your startup volume from HFS+ to APFS fails, and second, that key software proves to be incompatible with APFS (or another part of the upgrade).
How the upgrade works
Apple has been testing the tool which the High Sierra installer will use to convert your startup volume to the new APFS file system for almost a year now, and all the iOS devices which have been updated to iOS 10.3 have run that tool at least once, when they made that upgrade. Although this is a huge amount of testing and experience, you must be prepared for it to go wrong.
Beta releases of High Sierra have provided an optional let-out, which does not convert the startup volume to APFS. Apple’s latest published information on APFS conversion states that the High Sierra installer automatically and inevitably converts startup volumes on SSDs to APFS, but that startup volumes on Fusion Drives and hard drives will not yet be converted to APFS.
Apple’s conversion tool does the following:
- it first converts the existing HFS+ volume metadata (directories, etc.) to APFS format, and writes those metadata to free space on the disk,
- it then checks that the APFS metadata appear correct,
- if no errors have occurred, it then writes the volume superblock in APFS format, replacing the HFS+ data,
- it finally deletes the old HFS+ volume metadata, leaving the volume in APFS format.
You do not have to decrypt a volume which has been encrypted using FileVault: the conversion tool should seamlessly convert it straight to encrypted APFS format, using the same passphrase, etc.
Note that your startup volume must have sufficient free disk space to contain both its current HFS+ metadata and the replacement APFS metadata. Trying to convert a volume with insufficient free space is likely to throw an error, and increases the risk of disaster. However, it is not easy to predict how much free space will be required!
Belt and braces backups
For regular macOS upgrades, many users (including me) rely on their Time Machine backups in the event that the upgrade goes seriously wrong. It is laborious and time-consuming to downgrade to the previous release of macOS using such as backup, but unlikely to be a disaster.
Because of the increased risk of disaster when your startup volume is converted to APFS, you should not simply rely on one Time Machine backup to get your Mac out of trouble. I will be making a complete copy of my Home folder as well, and anyone with the time, storage space, and inclination would do well to make a mirror copy or clone of their startup volume immediately before installing the upgrade.
You should also run Disk Utility and its First Aid, preferably from Recovery mode, to ensure that your current HFS+ file system is in good working order. The conversion tool may be able to cope with some minor errors, but anything more substantial is likely to throw it out of kilter, making failure much more likely.
If Disk Utility cannot repair the problem, then you’ll have to reformat the disk, either straight in APFS prior to the upgrade, or before running the installer.
You shouldn’t upgrade until you are reasonably confident that all your most important apps – the ones which are essential to you – are likely to work reasonably well or normally. Compatibility of apps from major vendors like Adobe, Microsoft and Apple should become clear soon after High Sierra’s initial release, but there’s always the chance that software which you use less frequently, or which comes from a smaller vendor, may prove a problem.
One good way to cover this has been to keep a secondary Mac still running Sierra, so that if necessary you can revert to that when you need to run the incompatible software. Unfortunately, Apple’s announcement that APFS in High Sierra will be incompatible with Sierra’s APFS support presents a problem with such measures. Mixing Sierra systems with High Sierra APFS storage is not going to be good news, something which you will need to take into account.
There is always the possibility that Apple will recant, and provide an update to Sierra which enables it to access APFS storage fully.
Making the upgrade
Converting your startup SSD from HFS+ to APFS inevitably takes time. Don’t expect the High Sierra upgrade to be done and dusted in a few minutes, even on a fast, modern Mac.
Once you’re there, please send those of us with recent desktop Macs and Fusion Drives a postcard, letting us know how it went.