Time Machine backup almost full

If the Time Machine backup disk for your Mac is full - Apple Support

It will delete older backups to make room for new ones. It might be slow when it hits the storage limit. Ask yourself if you need the historical backups? Apple recommends disconnecting the full Time Machine disk and attaching a new blank one. Keep the original disk should you need to recover old data. Providing the long historical backups are actually necessary.

Some people may just decide to erase the Time Machine disk and reconnect it and set up Time Machine again because they don't care about the long historical data.

Boot your M1 Mac into Recovery and run a Disk Utility First Aid on your mounted Time Machine drive. If any errors are corrected, and First Aid ends with green status, that may make a difference in its backup behavior on a normal reboot and Time Machine run. If the pending backup is larger than what TIme Machine deems removable on the drive to accommodate the backup, then the backup will not occur.

In that case, it is either a new blank TM drive or whacking the current drive and starting over. There is no means to manually remove or move any content on an APFS formatted Time Machine drive.

The only reliable online instructions are Apple's, here: Back up your Mac with Time Machine - Apple Support, and the links within that document. For example is no longer possible to selectively "Delete all backups of <...>" which would have taken a long time anyway. If you find instructions suggesting that, they apply to very old TM versions. If you are using an M1 Mac you then you are not using one of those outdated versions.

If your TM backups are taking a long time to finish it's probably just busy deleting old and "expired" backups, which is to be expected when it approaches capacity. It does that after completing a new backup. If a great number of new items are added / changed on the source volume before the next scheduled backup and insufficient capacity remains to create a new backup, then it will need to do that first.

The likelihood of having to restore a backed up item diminishes with the passage of time. Once a TM backup drive has accumulated more than a few months worth of data I erase it. Of course I use multiple redundant backup devices, ensuring I am never left with only one.

It's normal for a sleeping Mac to wake for TM backups.

John Galt wrote:

The only reliable online instructions are Apple's, here: Back up your Mac with Time Machine - Apple Support, and the links within that document. For example is no longer possible to selectively "Delete all backups of <...>"

You're right. Unfortunately that feature was removed in Ventura. 😞

You might want to start over and replace Time Machine with Carbon Copy Cloner. It supports APFS snapshots and uses them to backup in a similar way to Time Machine and it gives you much more control over backups. Plus the author provides excellent support.

Time Machine is a whole different animal now that it uses the APFS filesystem w/snapshots on a physical disk. As a result, it is next to impossible to figure out storage size on snapshots vs reality or delete older portions of a Time Machine backup, etc. Entirely different than copying files and creating hard-links like it did with HFS+.

The eclecticlight.co blog has a ton of articles about Time Machine as well as some handy diagnostic tools. The author reverse engineered things and has documented all of it. If you are interested in what is going on under the hood, it's worth a read.

Historically, I've just erased an external physical Time Machine disk and started over. The long historical backups will only net you items that where deleted, overwritten a long time in the past. Starting a new backup takes your internal disk's current state and carries on from there. Everyone is different, some may need the historical backups for audit or compliance purposes.

Just to show what is possible at scale with Time Machine. Here is how I setup a a robust backup server for a departments Mac's before we started using 365 / OneDrive. Cloud has changed everything...

How much free space do you have on your boot drive? If there's not enough room for a local TM snapshot it won't backup.

You can check the local backups and remove them with Disk Utility. Always keep the latest one for safety's sake.

FWIW, there is no other alternative.

You're welcome.

If you are really serious about backups and maintaining a good backup strategy..?

Then like James, I recommend getting a second external backup drive and using CarbonCopyCloner.