I have been getting these messages for many months, and they are wrong:
I always backup when plugged in to wall power.
The backup always appears to complete -- that is the message in Time Machine says the most recent backup is just a few minutes ago (when I check).
Why? And (as this appears to me as a bug) why has it not been fixed?
MacBook Air, macOS 14.5
How much free space on both drives?
Start Terminal from spotlight.
At the terminal type tmutil listlocalsnapshotdates
Hit enter.
Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.
Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.
How many are shown?
Wrong backup error message
