I've got my NAS (FreeNAS) set up as a mapped network drive, and yesterday, after having made no changes to permissions or anything else in recent times got this pop up when attempting to copy across a video file (doesn't pop up until 95% of the file has been copied across):

As I say, I've made no changes recently, so wondering if it's a Windows 10 update snafu of some sort?

I've had a look at permissions (via properties on the target folder) in case anything needs changing, but nothing sticks out

Anyone got any ideas?