First Aid stuck at "Checking multi-linked files" on Time Machine Backup

Hello icemantx,

The time it takes to complete the First Aid process can vary depending on many factors like the size of your disk.

If you still have concerns with any delays in that process, let's use the steps below to boot your iMac into safe mode to see if either of those behaviors continue from there:

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.

2. Release the key when you see the login window, then log in to your Mac. 

3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

You can also find those steps in this resource: How to use safe mode on your Mac - Apple Support

It can take a few extra minutes to boot into safe mode. Your screen may flash or flicker while in safe mode. Those behaviors are expected. Testing in safe mode runs a check on your disk, removes some cached files, and can help isolate conflicts with startup items. Once you've tested that behavior in safe mode, you can restart to leave safe mode.

Let us know your results, and we'll keep moving forward from there.

I would let it run at least overnight. A 4 TB backup drive that is "nearly full" could take many hours to check because of the complex internal structures. Maybe even days. If it actually filled up the file system might be damaged, making it difficult or impossible to restore from.

Check Activity Monitor to see if data is being read from the drive and to see that Disk Utility is getting cpu cycles. Those would be encouraging signs.

If the backup drive is a mechanical drive, with APFS, and nearly full, that could be EXTREMELY slow when you try to check it with Disk Utility.

You could try to restore (not with Disk Utility running) by opening the backup drive in Finder and just using the Finder drag and drop type of copy to copy the file you want from the backup to your internal drive.

As for the internal drive, that can also be damaged if it falls below 10% free space if you are working on intensive things like videos. Have you checked the internal drive?

If you can't get a clean bill of health on the backup drive, it is suspect and I would leave it alone for now, and obtain a new drive and start a new backup set to it. Once you have reliable backups on the new drive, you can consider whether to try to erase/format the old one and continue using it. I would not use a drive for something as important as backups if you can't get Disk Utility to declare its file system healthy.

You can also run DriveDX on the external drive to check its mechanical condition. Run the extended (long) DriveDx test.