The error "Couldn't modify partition map because file system verification failed. (-69803)" and "An internal error has occurred. : (-69610)" may indicate that your external hard drive's partition map is corrupted, unknown or missing. This may be caused by incorrect operations, such as deleting the EFI system partition.
To fix it and be able to partition your external hard drive into your intended partitions, you can format your drive in macOS Recovery.
- Eject or unmount your external hard drive, then unplug it or turn off it and keep it plugged in.
- Restart your Mac, then press and hold Command + R immediately after your screen turns black. (M1 Macs have a different way to enter macOS Recovery.)
- Release the keys when the Apple logo appears.
- Go to Disk Utility > Continue.
- Turn on your hard drive or reconnect it to your Mac.
- Click View > Show All Devices.
- Select your external drive's physical disk and click Erase.
- Run First Aid to repair the disk and then the volume.
- Repartition your drive.