Time Machine backup failed - Error 21

Unfortunately, we've provided you with the best resources available.

Hold down the Option key on the iMac while you click on the Time Machine "clock" icon at the top of the screen. Click Verify Backups to see if the procedure will run.

If no luck, hold down the Shift key on the iMac while you click on the Time Machine "clock" icon. Click Back up with Consistency Scan to see if things will run that way.

If still no luck, my suggestion would be to delete the sparsebundle backup file associated with the iMac and start over again. If you don't want to lose all the old backups......which are likely corrupted......you can use the Archive function in AirPort Utility to copy over all the data on the Time Capsule to a USB drive. But, since the iMac backups are likely corrupted, I doubt that they will be of any use going forward.

Click on the "smiley face" Finder icon on the dock at the bottom of the Mac's screen

Click on the Time Capsule icon under the Shared heading on the left panel of the Finder Window

A folder named "Data" (unless you have changed the name of the Time Capsule disk) will appear

Double-click on Data to open the drive (If you have not done this before, you will likely be asked for the Time Capsule administrator password)

Click on the sparsebundle file with the name of the Mac to highlight the file

Click the "gear" icon at the top of the window

Click Move to Trash

The operation will take a few minutes to complete

See #C17 in http://pondini.org/TM/Troubleshooting.html

The first two failed

This would confirm that the backup file had become corrupted......something that occurs far too often with users who are running Sierra on their Macs.

Just a quick question: where do I find the sparsebundle backup file so I can delete it?