Hi
I have had this issue with a quite a few system builds now not always just with this particular model
about once every 2 months or more the bios resets some or all of it settings.
And it only seems to happen to systems with removable drives permanently connected or card readers.
Updating the bios hasn't always helped. or turning off legacy USB.
however in 1 case with a Gigabyte GA-H77M-D3H updating the bios did in fact fix the issue.

Here's What i think might be happening
The USB storage device doesn't respond to the bios querying it in a timely manner for what reason so it the bios resets all the USB and drive related settings to try correct this for the next boot.

The current system with the issue has a Kingmax SSD for the system drive and a WD Black Edition for Data the removable USB HDD is a older one possibly a seagate
Regards,
Chris