This problem coincided with the latest Windows 10 Anniversary update (vers. 1607).
The PC is also having difficulty booting up since the above upgrade and I frequently see the message:
"Reboot and select proper boot device or insert boot media in selected device and press a key"
I have tried a whole host of suggestions including one from Microsoft Support KB3083595 that disables storAHCI MSI through the registry that didn't seem to make any difference.
I did notice (too late as it was after I upgraded) that ACER don't support the Windows 10 upgrade at all for this model but they don't seem to give a reason.
Any help / suggestions appreciated.