What to do if LaCie external drive not mounting on Mac?

According to what you've mentioned, the reason why you encounter the "File system verify or repair failed. : (-69845)  Operation failed" error on your Mac probably is probably because of the frequent unsafe ejection. In this situation, the file system/partition map of the external drive may be corrupted and 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 (you've mentioned it already). But reformatting will wipe all data off the disk!

Though you've said the information on this drive is not terribly important, you'd better use iBoysoft Data Recovery software to scan it first to avoid if you've forgotten there are some important files on it.

This easy-to-use macOS program can scan the corrupted/damaged external drive and search for all files on it, after you click the "Scan for Lost Data" button, all the files on this drive should be demonstrated. Then, you can check if there is any vital data on it, if there is, simply hit the "Recover" button to recover and store it in another healthy place.

Now you can 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 fixed and the disk should be able to mount in your Mac correctly. If your drive failed to be connected with the Mac before reformatting. Check the physical connection first and then force quit fsck_ext from the Activity Monitor.