System Recovery 2013 R2 SP3 (18.104.22.168088) on a clean install of Windows 10 Pro 64-bit
I'm backing up to an external hardrive connected by USB3
9 times out of 10 the backup hangs at the message 'Snapping volumes', and there doesn't seem to be a way to cancel it nicely.
I don't see any error message.
I was also getting the same now and again on Windows 8.1, but it nows seems more frequent.
In case you were not aware, Windows 10 is not officially supported yet:
You may want to check the Windows event logs (system and application) for errors. If you reboot the machine, does the backup work after the reboot? If yes, I suspect some sort of VSS related issue here.