To fix the error reading "An internal error has occurred (-69610), "try to erase your external hard drive in macOS Recovery. Note that this will remove all data on the drive. So back up your files before proceeding!
- Eject your external hard drive.
- Reboot your Mac and press Command-R when the screen turns black.
- Release the keys when the Apple logo shows up.
- Connect the external drive to your Mac.
- Open Disk Utility.
- Click View > Show All Devices.
- Select your external hard drive and click Erase.
- Name your drive.
- Choose a proper format.
- Keep the scheme GUID Partition Map.
- Click Erase again.
- After that, you can run First Aid on your external drive again or repartition it as you like.
