External Disk not recognized, can't unmount, cannot erase I have an external drive which is only recognised running diskutil list in Terminal, not in the GUI version or Finder:

/dev/disk1 (external, physical):
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *500.1 GB   disk1
1:                        EFI EFI                     209.7 MB   disk1s1
2:                  Apple_HFS Backup                  250.0 GB   disk1s2
3:       Microsoft Basic Data XHDD                    249.6 GB   disk1s3


I have tried the following:

diskutil verifyDisk disk1


Problems were found with the partition map which might prevent booting
Error: -69808: Some information was unavailable during an internal lookup

diskutil verifyVolume disk1sn
diskutil repairVolume disk1sn


Error starting file system verification for (bad disk ref): Disk object invalid or unable to serialize (-69810)

diskutil repairDisk disk1


Problems were encountered during repair of the partition map
Error: -69808: Some information was unavailable during an internal lookup

diskutil eraseDisk JHFS+ Untitled disk1


Error: -69888: Couldn't unmount disk
or:
Error: -69877: Couldn't open device

For the first error I did subsequently:

diskutil unmountDisk force disk1


Which most of the time was sucessful. If not, I even tried to find processes that interfered with the drive (found here: stackex/os-x-cant-unmount-disk)

sudo lsof|grep disk1


Which did not find any processes.

In fact, now i can't even get to the unmountable part and alwas get the:  

Error: -69877: Couldn't open device

Is this thing really done or is there more to do here?
When the command line cannot erase the disk, it’s time to reseat and replace all cables or take that drive to another Mac to eliminate a hardware failure. 

You are using the correct erase syntax so this is major macOS side failure (unlikely but certainly possible) or hardware failure (likely).