On my seventh attempt to get this farce of an update to install I reset the computer. When I had originally upgraded to W10, I promptly re-installed after activation; so my reset was to that fresh install, on which I performed sfc /scannow to check its integrity before attempting to install the update, using a USB created using Windows Media Creation Tool. I also uninstalled AntiVirus program, and had no USB devices connected except the one containing the update.
This resulted after 4 hours and an almost completed operation in Windows reverting to existing version with Error Code 0x8007042B-0x4000D "The Installation failed in the SECOND_BOOT phase with an error during MIGRATE_DATA operation". I then spent a happy(!) couple of hours restoring from image backups to where I had been before performing the Reset.
I know from a questionner on Microsoft Support that a clean boot does not do the trick.
What I fear is having that abortion of an "update" downloaded automatically when I am busy using the computer for work (I am using Windows 10 Home, so cannot pick and choose updates) and cannot spare another 6 hours on Microsoft's cockups (British English!). I cannot afford that, so I have disabled Windows Update in services.msc. However, it is not safe doing without updates for any length of time, so ideally I need a senior Microsoft expert who actually knows what he is doing, who instead of trotting out formula answers (such as suspend AntiVirus or clean boot) can give chapter and verse of how to make this thing work. Either that, or existing Windows 10 users should be able to download an ISO of the 1607 build to install from scratch.
I need an answer as fast as possible so I can resume protecting my computer with updates without risking downloading what is effectively junk.