Updated the MBA to macOS 10.13.2 the connected a USB drive in order to transfer the updater to the iMac.
Up popped a warning that the system was unable to repair the attached drive.
I tried copying the updater to the drive anyway but copying was impossible.
Fired up DiskWarrior which found heaps of problems with the directory. So I replaced said directory with the new one that DiskWarrior has created.
All fixed: can now copy to that ext drive.