Time Machine Backup Fails

Sorry, but we cannot tell from the information in your post whether or not you are backing up to a Time Capsule or to an external drive that is connected directly to your Mac via USB.

It is a bit early to jump to conclusions, but if you are backing up to a Time Capsule, it sounds like the backup file may likely be corrupted. When this occurs, you will usually......but not always.....see a message like the one just below:

If.....you are backing up to a Time Capsule, about how old is this device?

If.....you are backing up to a Time Capsule try holding down the Shift key on your Mac while you click on the Time Machine "clock" icon at the top of the screen. Select Back Up With Consistency Scan if it is available.

I tried Disk Utilities and got the message OS X can't repair the disk "Time Machine Backups".

Disk Utility cannot repair Time Machine backups if you are backing up to a Time Capsule. Disk Utility will only work as far as repairs on a drive that is connected directly to the Mac, like a USB drive.

I am actually backing up to an external hard drive that's directly connected to my iMac.

Then, Verify Backups or Back Up With Consistency Scan are not available for this type of setup. These checks are only active and selectable if you are backing up to a Time Capsule.

About all that you can do is try running Disk Utility again a few times to see if the disk can be repaired. If it cannot, then the backup file is corrupted.....or.....the disk itself may have a physical problem.

I assume that you have already tried powering the external drive off a few times and restarting your Mac a few times, correct?

We have seen a number of backup errors reported from users who have "upgraded" their Mac's operating system to Catalina.

Which operating system are you running on your iMac?

Does the external hard drive appear on the iMac's desktop? If yes, right-click on the drive and click on Get Info.

What is the format of the external drive?

Capacity?

Available Space?

Used Space?

Next, look at the Sharing and Permissions section near the bottom of the information window. What information appears as far as permissions?

So far, everything that you are telling us indicates that the WD external drive is OK, but the Time Machine backup file on the drive is corrupted. However, if the WD drive is 6+ years old or older, it is now suspect......even if it appears to be working OK. I would not keep any data on the drive that you cannot afford to lose.

As far as getting Time Machine backups going again......

1) The simplest thing to do is add another external hard drive and start over again with Time Machine backups. Keep the old drive if you value the data in the old Time Machine backups.......even though they may not be accessible any longer. If the drive is getting old......6+ years is getting old for a hard drive......this would be a good option.

2) A bit more complicated.......copy the Time Machine backup file over to another hard drive......if you want to try to preserve the backups.....even though they may not be accessible any longer. Then erase the current external drive and start over again with new backups. I would not recommend this though, if the external drive is getting along in years.

3) Simply erase the current external drive that you are using and then start over again with new backups. You lose your old backups this way, but they are not likely useable any longer anyway. But, you would not want to do this if the drive is getting up there in years. Buy a new drive in that case.