Time Machine "Backup Not Completed" error after installing Microsoft 365

You have solved the problem. I was able to free up space on the drive and it is now backing up. I really appreciate the help you have given me. The installation of Microsoft 365 must have tipped the scale just enough so that there was not enough space to backup.

Sikh wrote:

https://discussions.apple.com/content/attachment/2deb350d-d7e8-437f-a288-41aea6173772

You may be right. It appears there is only 14 GB left. I am going to try and free up some space tomorrow and see if that solves the problem. Thanks for the suggestion. I will let you know how it works.

14 GB is only about 1.4% of 1 TB. So the Mac needs about 15% FREE SPACE to function properly. Your backup probably couldn't stage the backup properly with only 1.4% free space (14 GB). If you are using any Adobe apps, those won't function properly with so little free space. Also, I don't know how memory you have, let's just say it is 32 GB memory. When your Mac needs swap space because it has used up all memory, it will start paging to disk but with only 14 GB free, it will slow down considerably as it looks for space which does not exist. If Time Machine is also running, that needs memory and scratch space as well and the backup might simply stop to not conflict with other programs running.

See if you can free up at least 150 GB space which needs to show as FREE SPACE in Disk Utility.

Sikh wrote:

I am getting a Time Machine "Backup Not Completed" Error. I have an iMac Retina 5K, 27inch 2020 running Tahoe 26.7.1. It started right after I installed Microsoft 365. I have tried the following. I erased the Backup drive and reformatted it and set it up as the Time Machine drive. I turned off Microsoft OneDrive and also stopped it from loading during startup. I have restarted after each attempt and I still get the error. Any suggestions?

Right after installing MS-365 ... the combined space taken by your new installation of the MS 365 applications is 10-15 GB. Additional space could be required for other associated files from MS-365 placed in other folders (I just checked my Applications folder where I have the MS-365 applications). Also, if you were using MS OneDrive that could be using some amount of storage as well. For Time Machine backups to work, you need scratch space available on your internal drive for Time Machine to properly stage the backup. You also need adequate space on the backup target drive, which I suspect you do have since you have a freshly erased drive for that now.

How much FREE SPACE do you have on your internal drive? The only sure way to answer that question is to open Disk Utility and see how much space it shows as FREE (not "available," as FREE is not equal to "AVAILABLE"). Also, how big is your internal drive? How big is the target backup drive?

You may be right. It appears there is only 14 GB left. I am going to try and free up some space tomorrow and see if that solves the problem. Thanks for the suggestion. I will let you know how it works.

Glad it worked out. Going forward you may have to watch storage use and adjust accordingly.

Once you reach a point where you cannot delete files anymore there are two approaches:

(1) Move files to be stored in the cloud, freeing up local space on the Mac. Apple offers iCloud storage for this, and there are other options as well (Dropbox, Google, Microsoft). The Apple iCloud integrates well with the macOS. Microsoft's One Drive integrates with use of MS-365 software. One downside to this is that most backup programs (including Time Machine) only back files that are physically on the disk being backed up; files that are physically only in the cloud are not backed up. However one program, Carbon Copy Cloner, can be configured to back up files that are in the cloud, but this requires CCC to download and then copy those files, and no doubt this takes additional temporary disk space during that process. Another downside to cloud storage is that access to those files depends on the internet connection, they have to be downloaded to be accessed.

(2) Buy additional local storage (external drives) and move files to those external drives from the internal drive. Here is one example: Move your Photos library to save space on your Mac - Apple Support

However when you do this, those files on the external drives also have to be backed up.