If you bought an M1 Mac with just a 256 GB internal SSD and have kept up with macOS upgrades and updates, should you be worried that it’s running out of space by the time it makes it to Tahoe? Dare you look at Storage settings to see how much of the SSD is now swallowed up by System Data? This article explains why macOS 26 shouldn’t devour the last of your SSD, and how you can ensure that it doesn’t.

What’s on your Mac’s internal SSD?

Internal boot disk layout is most complex in Apple silicon Macs, as theirs is divided into three partitions (or APFS containers). Two are hidden and contain pre-boot and other low-level support files, and amount to around 6 GB. The Macintosh HD partition then takes the lion’s share, the whole of the remainder. Even on a 256 GB SSD, that’s about 250 GB.

Volumes within Macintosh HD include:

• System, just over 12 GB,
• VM, varies in size according to how much virtual memory is swapped out to disk,
• Preboot, just under 8 GB,
• Recovery and others not normally mounted, a total of less than 2 GB,
• Data, whose size is determined by what you store there.

The system your Mac actually boots into isn’t the System volume itself, but a snapshot made of it, occupying the same space, plus a little extra for the snapshot’s metadata including its tree of hashes to form its seal and signature. Because this is a snapshot it uses the same data stored for the System volume, and doesn’t double that up.

This should allow your Data volume a maximum of 228 GB, less any space required by the VM volume. Although installation of a macOS upgrade or update will require substantial additional space, once that’s complete the space taken by the System volume and its snapshot should fall to little more than 12 GB.

What happens when macOS is upgraded?

In traditional macOS upgrades, the Installer app was downloaded first, and itself required around 13-15 GB. That was run, and expanded its contents to be installed onto the System volume, replacing much or all of it.

Updates work more economically, as they contain only the files that have changed, so far less than the Installer app. When they’re installed, they replace only those files changed in the System volume, ready for a new snapshot to be made from that, to be used to boot that Mac. So an update-style upgrade, as you should get when going from macOS 15.7 to 26.0, should require a much smaller download, a faster install, and less space to install the new version of macOS. However, the end result should be identical, with exactly the same files installed in the System volume, and exactly the same in the snapshot used when running.

Whichever is used, the installation process is similar. First, the files to be installed are expanded, then they’re written to the mounted System volume, with some going onto the Data volume as well. Once the System volume is complete, a snapshot is made of it, and that’s sealed using a tree hierarchy of hashes, culminating at the top of the tree in the seal.

What is System Data?

Storage settings scans the contents of the boot volume group, Macintosh HD, and divides the storage used into different categories like Applications and Podcasts. It appears to total those up and account for the remainder of storage used in the category System Data. That doesn’t include the size of the System volume, or its snapshot, but can include temporary files like caches, snapshots, and anything else it can’t account for in other categories.

Taking control

If there are substantial amounts of space that aren’t accounted for on your Mac’s internal SSD, and you want to reduce that, you need to account for it before deciding what to do about it.

First check for large snapshots. I hear repeatedly of Macs that turn out to have hundreds of GB being used by snapshots unnecessarily, and the current record is over 400 GB. The easiest place to check for those is in Disk Utility. In the sidebar on the left select the Data volume, then Show APFS Snapshots in the View menu for them to be displayed at the foot of the main view.

Backup utilities including Time Machine normally make a snapshot with each backup, and retain them for 24 hours, following which they’re automatically deleted. As snapshots can’t exclude folders in the way that Time Machine can in its backups, if you’ve been working with a couple of 100 GB VMs then they will be retained in snapshots even though you probably exclude them from being backed up.

Once you’re happy that free space isn’t being retained in snapshots, use a disk mapping utility like DaisyDisk or GrandPerspective to hunt down other large files and folders that you may not need. One reader here recently discovered that their iOS and iPadOS backups had taken over more than half the space on their Mac’s SSD.

Wait a day or two after upgrading

Installing a macOS upgrade also changes files on your Data volume, and may retain temporary support files. These are normally cleaned up in the next 24 hours, and you may be able to encourage that by starting your Mac up in Safe mode, leaving it a couple of minutes, then restarting it in normal user mode.

By a couple of days after the upgrade, your Mac should have returned to normal use of storage. If it hasn’t, check snapshots and go hunt that missing space.

No sooner have we recovered from upgrading and updating macOS to 26.0/15.7/14.8 than Apple has released the next round of betas. This article looks at what’s in store for us over the coming year, as far as macOS is concerned.

With pandemics hopefully behind us, Apple’s planned OS updates have settled into a more regular pattern. Release dates when Sonoma was the current version of macOS (2023-24) were:

• 14.0 – 26 September
• 14.1 – 25 October
• 14.2 – 11 December
• 14.3 – 22 January
• 14.4 – 07 March
• 14.5 – 13 May
• 14.6 – 29 July
• 14.7 – 16 September.

Over the last year (2024-25), Sequoia has been almost identical, allowing for the small vagaries resulting from our calendar:

• 15.0 – 16 September
• 15.1 – 28 October
• 15.2 – 11 December
• 15.3 – 27 January
• 15.4 – 31 March
• 15.5 – 12 May
• 15.6 – 29 July
• 15.7 – 15 September.

If Tahoe follows the same pattern, you can expect releases to occur on the following dates:

• 26.0 – 15 September 2025
• 26.1 – 27 October 2025
• 26.2 – 15 December 2025
• 26.3 – 26 January 2026
• 26.4 – 30 March 2026
• 26.5 – 11 May 2026
• 26.6 – 27 July 2026
• 26.7 – 14 September 2026.

If you’d like a week’s notice of scheduled updates, watch Apple’s Developer Releases newsfeed at feed://developer.apple.com/news/releases/rss/releases.rss for Release Candidates. For minor versions, those are normally released about a week before the intended final release, so RCs seen on 20 or 21 October are likely to be followed by the public release on about 27 October.

Those can of course slip a few days or even a week if there are serious problems remaining with a release candidate, and some may be rescheduled to coincide with hardware announcements. These are also the ‘minor’ version updates, and Apple is likely to intercalate ‘patch’ releases to fix any serious bugs or urgent security vulnerabilities. Those almost never go through beta-testing or release candidacy.

For those staying with Sequoia or Sonoma for the time being, those security updates are most likely on the same dates as those for Tahoe.

Finally, a reminder for those whose Macs are still running macOS 13 Ventura: the final security update to 13.7.8 was released on 20 August this year, and Ventura is no longer officially supported by Apple. If your Mac can run Sonoma or later, and you want continuing security updates, then you’ll need to upgrade it to Sonoma 14.8 or later.

If you have a Mac Studio M3 Ultra and want to upgrade it to run macOS 26.0 Tahoe, then I’m afraid you’re going to have wait for Apple to build a new release that will install on your Mac.

I’m very grateful to Ken who has tried unsuccessfully to upgrade from 15.7 to 26.0. There are plenty of others reporting exactly the same: the upgrade goes well until towards the end, then aborts and the Mac is restarted back into 15.7. The problem seems to originate from an error in its neural engine driver.

Having just taken a look through a comparison between kernel extensions shipped with macOS 15.6 and 26.0, there are several Apple silicon hardware kexts that seem to have gone missing in 26.0, although whether that’s the cause only Apple’s engineers should know.

Apple is advising all those affected to put their Tahoe upgrade on pause until it releases a new build that does fully support the M3 Ultra. Until then, 15.7 is the limit for Apple’s most powerful and expensive Macs yet.

Apple has just released macOS 26.0 Tahoe (build 25A354), together with security updates to Sequoia taking it to 15.7, and for Sonoma to 14.8. As expected, there are no further security updates provided for Ventura, which is now unsupported.

The upgrade to Tahoe is once again provided as an ‘update’ rather than a full Installer app. If you want to run the Installer app to upgrade, download it from the App Store rather than using Software Update. If you’re updating Sequoia or Sonoma and your Mac is capable of running Tahoe, be very careful to select the right update in Software Update.

The Tahoe upgrade weighs in at 7.7 GB for Apple silicon Macs upgrading from a recent version of Sequoia. For Intel Macs it should be 6.1 GB.

On Apple silicon Macs, iBoot is updated to version 13822.1.2. Intel Macs have their firmware updated to version 2092.0.0.0.0 (iBridge 23.16.10350.0.0,0). Safari is version 26.0 (21622.1.22.11.14). The Darwin kernel version is 25.0.0.

Security release notes are also available:

• Tahoe 26.0 lists 75 vulnerabilities fixed, none of which is reported as already being exploited.
• Sequoia 15.7 lists 34 vulnerabilities fixed.
• Sonoma 14.8 lists 38 vulnerabilities fixed.

Useful links

Prepare to upgrade macOS – what you should have done already
What should you do when an update goes wrong?
When you should use Safe Mode, and what it does
What to do when there’s something fundamentally wrong with an Apple silicon Mac
Eclectic Light software updates for Tahoe

Last updated at 1928 GMT 15 September 2025. My apologies for some previous incorrect versions, which were the result of an unintended update.

Apple has announced that macOS 26 Tahoe will be released on Monday 15 September, slightly earlier than had been speculated. Even if you’re not intending to upgrade to that, you might instead be looking at moving from Sonoma to Sequoia, or perhaps dragging your feet and considering Sonoma as it enters its final year of support. This article considers what you should do when preparing to upgrade macOS.

One of the surgeons I worked for in my first internship in hospital taught me an important lesson in life: when considering the outcome of anything that could go wrong, assume that it will go wrong, and prepare for that. When it actually works out better than you planned for, you can enjoy your success.

Emergencies

The worst case is that your Mac dies during the upgrade. Although that’s also the least likely, you need to think through your disaster plan. I ensure that all my most essential files and data are shared or copied up to iCloud so that I could get by for a day or three without that Mac. A recent full backup is also essential: if your Mac needs to go away to be resuscitated, one way or another that’s what you’ll be restoring from.

Upgrades do bring a tiny but significant risk of bricking your Mac in a way that only a full Restore will recover it. Although this can apply to Intel Macs with T2 chips if a T2 firmware update goes wrong, this is more the preserve of Apple silicon Macs. I’ve recently stepped through your options with full details here. Your first DFU Restore is daunting, but once you’ve done one, you’ll realise that they’re not that challenging if you have the right cable and DFU port. When you’ve restored firmware and macOS, you’ll then be restoring from that last backup, emphasising its importance.

In the days before the SSV, when there was only one boot volume and that could so readily be corrupted during upgrades, you also needed to have an emergency toolkit handy to repair an upgrade that went wrong. These days, the whole of the System in the SSV is either perfect, or macOS has to be reinstalled. Minor glitches are almost invariably corrected by restarting after the upgrade has completed, or starting up in Safe mode (remember on Apple silicon Macs that’s performed from Recovery).

Reverting macOS

The other possibility that you should plan for is beating a hasty retreat and reverting to an older version of macOS. Provided that you’re fully aware of the changes to the macOS interface brought in Tahoe, I think this is less likely for those upgrading from Sequoia, but if you’re skipping a version or two you could still find yourself unable to use a vital peripheral or one of your key apps, leaving you with reversion as your only option.

I’m sometimes asked by eternal optimists whether you can revert to your previous macOS simply by using its SSV snapshot. Sadly, snapshots are of no help: the only way back is to wipe and reinstall that macOS.

On Intel Macs, you’ll need to do this when booted from an external bootable installer, which doesn’t have to be on a USB ‘thumb’ drive, but does still require its own HFS+ volume to work. Apple explains this here, and Mr. Macintosh has links to all available installer apps.

Although you can do that with an Apple silicon Mac, if you have a second Mac and the right USB-C cable, it’s usually quicker and simpler to do this by restoring from the appropriate IPSW file in DFU mode, then restoring your files from your latest backup, as explained here. This is particularly valuable, as it also restores the original firmware, which may be the root of your problems. Unfortunately, that doesn’t seem possible with Intel Macs. Once their firmware has been upgraded, the user isn’t able to downgrade it.

Checklist

• Check you’re prepared to use your disaster plan if needed.
• Consider sharing and copying to iCloud to help you use another Mac or device temporarily.
• Make a full backup immediately before starting the upgrade.
• Restart, or start up in Safe mode, if the upgrade leaves your Mac with problems.
• Reverting to an older macOS isn’t trivial, and will require you to restore from your backup.
• Revert an Intel Mac using a bootable external installer.
• Consider reverting an Apple silicon Mac by restoring it in DFU mode, using an older IPSW.

Whatever you choose to do, I wish you success, and hope that your preparations prove completely unnecessary.