Every iPad sync ends with - Attempting to copy to the disk Macintosh HD failed unknown error -4

I started getting failures of this nature quite some time ago, though before that the iPad synced fine. I'm trying to sync to a latest model iMac.

The iMac was set up from the backup of my old iMac which died. Not sure if that's the cause of this. I have tried the file verification process I read about on another post, but after running it to completion it made no difference to the error.

This issue has continued through each upgrade since it started happening. Both iMac and iPad are on the latest OS.