Time Machine fails to back up after updating to Tahoe 26.2

Metal_bender wrote:

Updated to Tahoe 26.2. Now Time Machine fails to back up. (Backup failed). Is there a 'fix' or should I delete all backups & start over?

Are you presented with a reason for the failed backup?

What is the capacity of your Time Machine backup drive?

If the backup drive is too full then the backup cannot be completed. If this is the case, your options are wipe the drive and start a new backup, or add a second backup drive and shelve the full one.

What is the capacity of your startup drive and how much free space do you have?

A too full startup drive may confound the backup process. If this is the case, you'll need to clear space on the startup drive. Your startup drive should always have 10%-15% of its total capacity as free space.

Metal_bender wrote:

I did wipe the back up hard drive. Took a while, but OK. Then changed the name (only), and specified the back up drive.

Now it works, smoothly. I didn't feel comfortable (a time period of no backup or ™ access), but the machine & I got through it.

Happy to know you got this sorted out. Thanks for the update.

I hope Apple Tahoe developers are checking out _all_ the possible problems us users detect.

Never gonna happen if one understands the math of it.

Many millions of Macs x many millions of apps + configurations = IDIC (Infinite Diversity in Infinite Combinations)

😉

Metal_bender wrote:

The most recent backup (using previous macOS) is 1.29 TB. Available space is 1.33 TB. I"m guessing this is the key reason for 'Backup failed" (with no further explanation).

That would not be my assumption.

A macOS update should not cause a backup to fail, especially when it appears that you have plenty of space available on that TM backup drive. Unusual stuff does happen from time to time, though.

What threw me was, that this is virtually as soon as the macOS was upgraded. Can we hypothesize taht backups with Tahoe 26.2 cannot delete older backup files?

Yes, we can hypothesize that, as it seems there may some evidence of that historically. I'm of the opinion that if there are very old files remaining in place on the Mac, then TM cannot delete the oldest backups of those files and so cannot free up space in that manner.

Conclusion:

Wipe the backup drive & start fresh.

This is a reasonable conclusion, imho. If you are confident that the backup contains no files that are not on your Mac, then it is certainly the easiest and fastest remedy for a misbehaving TM backup process. I'd do that.

For future reference, should you come across Time Machine related issues, I have written the following user tip. You my find it helpful in narrowing down, for each phase of the Time Machine backup process, what the issues, and their associated potential culprits, are.

Time Machine Troubleshooting - Apple Community