What to do external drive not mounting and First Aid failed?

When you encounter the "File system verify or repair failed. : (-69845)  Operation failed" error on your MacBook, it means your external disk has some problems that may be caused by sudden power failure, catalog file corruption, invalid volume header, virus attacks, etc. In this situation, you should repair your disk first otherwise you can't either mount the drive or access it.

Since the Disk Utility First Aid failed to repair your disk, the last method you can use to fix the "File system verify or repair failed in Disk Utility" issue is to reformat it. But reformatting will wipe all data off the disk! Therefore, if you got any backups of the external drive, you can start formatting right now. If no backups, it's recommended to recover files from the drive with a data recovery tool first.

There are many data recovery apps in the market, here you can use the iBoysoft Data Recovery for Mac app to recover data when the external hard drive not mounting on Mac:

Step 1: Download install and launch iBoysoft Data Recovery for Mac.

Step 2: Select your external drive from the Storage Device list and click Search for Lost Data.

Step 3: Preview the search results and select the files you need and click Recover to get them back.

After successfully recovering data from the disk that's not mounting on Mac, you need to format the external drive on Mac. When finished reformatting, try reconnecting the external drive again, the "File system verify or repair failed in Disk Utility" issue should be resolved and the disk should be mounted in your MacBook Disk Utility correctly.