Try Resetting your TimeMachine backup. Worked for me. This is from my notes.
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Time-Machine Backup - Reset and Start-Over (good to do any time OS is upgraded)
Note: DO NOT just toggle TimeMachine Backup OFF, and then just delete files from Backup_500. They will go to Trash and there will be too many to delete later. Erase drive instead.
If you have a "Archive and Backup Files" folder on it (in root) that you manually copy stuff too, copy it to desktop or somewhere
safe temporarily until this procedure is finished.
Toggle Time-Machine Backup OFF from Top-Menu (uncheck Backup Automatically).
Click "Select Backup Disk" button and Unselect existing drive
Use SpotLight Search or otherwise start Disk Utility
Select the TimeMachine hard drive (top level) from the left menu, then choose the “Erase” tab
Select format type “Mac OS Extended (Journaled)”, give it a logical name (like Backup_500), and choose “Erase”, confirm to erase at the next screen.
After it finishes, if you select Partition tab, you will see it is one large Partition, the Map Scheme is GUID Partition Table
- and other options you just selected
Quit Disk Utility
Start TimeMachine
Be sure "Old Drive" is unselected
Select new drive (Backup_500) from Available Drives, and click Use Disk button
Turn on TimeMachine (be sure Backup Automatically is checked)
Within a few minutes, it will start backing up "Macintosh_HD" to "Backup_500".
- This initial backup might take a while because its backing-up everything.
Yes, "Backup_500" must-be/should-be excluded from backup because if not, it would try to backup itself in a loop.
As a final test (and one reason TimeMachine was erased and reset in the first-place) be sure that the
Used space of Apps (usually first in blue on the bar) is ballpark-correct in About This Mac/ Storage.
Copy "Archive and Backup Files" folder back to root of drive.
Done and working.
