For the last 38 days, I have been measuring free and used space in my Mac’s boot container and its backup storage. Here’s what has happened, and how upgrading to macOS 27 changed it.
free space
Although they can only grow in size, and mustn’t be left lying around, snapshots can be very useful, provided you manage them carefully, as described here.
How to put your Mac into Safe mode, what Apple claims it does, and what evidence there is of its benefits. Includes an assessment of free space it may create.
What happens when a Mac runs out of free space in the container of its boot volume group? This shows what you can expect to see, and what happens in the log.
You’re running out of free space, so check in Storage settings to see where it has all gone. System Data has reached 50 GB, but what is that, and what should you do to reduce it?
The Finder’s Get Info figures allow for sparse files when giving size ‘on disk’, but don’t take into account shared data in clone files. This can prove misleading, as shown here with VMs.
Major improvements in its plots make this new utility even better for tracking changes in used space, and so forecasting how long backup storage will take to fill.
Snapshots made by Time Machine and tmutil should be purgeable, although sometimes they won’t purge, and aren’t shown reliably as purgeable space. An exploration of bugs and missing documentation.
All APFS volumes in a single partition/container share its free space. How can that be measured? How much could be purged, and how do special file types affect it?
To work out how long you’ll be able to use your present backup storage, you need to know how much space is free now, and the rate at which free space is being used by further backups. This app can forecast that, and more.
