Windows 8.1: System Image Backup of C: drive fails, Error 0x8078012D and 0x8007045D

Hello everyone.

I've a lot of people encounter problems similar to this, but here is my particular flavour:

I have a Windows 8.1. Pro (64 bit) desktop PC.

I am trying to use File History > System Image Backup to create a backup image of my C: drive.

I have been trying to save the backup both an external USB 3.0 drive, and this time to a network location (on a WD 'MyCloud' EX4 NAS)

Every time, the backup fails about 3/4 of the way through.

I get an error dialog saying

"The backup failed.  The operation failed due to a device error encountered either with the source or the destination. If the source or destination volume is on a disk, run CHKDSK /R on the source or destination volume, and then retry the operation. (0x8078012D)Additional Information: The request could not be performed because of an I/O device error. (0x8007045D)"

I've spent many hours diligently running CHKDSK /R  both on my C:\ and the destination drives, and it has never once found any problems.

I've tried closing all applications, unnecessary services, background processes and disabling Windows Defender's real time malware checking whilst the backup takes place, but that doesn't seem to fix anything.

My hunch is that there's nothing wrong with the disks or hardware at all, and that this is some kind of obscure permissions problem, where Windows can't access symbolic links on my C: drive.

But I basically haven't a clue! :) 

I'd be very grateful for any suggestions at fixing this, as currently I don't have any backup image of my C:\ drive, so if my PC goes kaputt, I'll have days of manual re-installing and configuring to do.

Thanks.