I have a Buffalo NAS drive for my backup data with mine and my wife's PCs backing up data to it on a daily basis.
On Monday 9 March 2015 my backup failed to run. Turns out it could not access the NAS drive. Thought it was the NAS Device, even upgraded firmware to no avail.
Checked from my wife's PC and it had full access to the NAS drive.
Checked my PC and found that a number of Windows updates were applied on 9 March 2015.
Restored my PC back to an image I had taken on 6 March 2015 and the NAS drive was accessible again.
Turned Automatic Update off to ensure I didn't apply it again.