Disk Utility restore failed - inverter failed
I'm running macOS 10.13.4 and my file system is APFS.
I want to create a backup on an external hard drive - Seagate GoFlex for Mac. I have often done this previously using DiskUtility in Recovery mode. The external hard drive was HFS+.
When I tried using the latest DiskUtility it failed to verify the target saying not enough space.
After various failed attempts to sort things out I eventually used diskutil from the command line to eraseDisk as APFS. I went back to Recovery mode and started to restore my system to the external hard drive using DiskUtility.
All seemed to be going well (but slowly ! - it took 7 hours) and verification seemed OK only for it all to fail at the last minute.
The error message was "APFS inverter failed to invert the volume" saying there was an invalid argument.
I'm left with a single large file (500GB) named ContainerToInvert.
Is there any way to complete the inversion from this point?
Any ideas what went wrong?