[UPDATE 04/08/2016 -- Performed a full PC Reset (wiping all data) using the Recovery options in the Windows 10 Settings. New issues have become apparent. Please check my reply/post for this date for more information.]
Today, I ran a virus scan which returned two errors while checking the aforementioned folders.
Upon trying to access the folders myself, the following was returned (respectively):
C:\Users\All Users is not accessible.
The device is not ready.
C:\Users\Default User is not accessible.
Access is denied.
Meanwhile, the antivirus I was using returned the same "The device is not ready" error for both folders.
My computer is less than 3 months old, and used solely for gaming. I store my games on a 1TB HDD, while the system files, OS and anything else is stored on a 240GB SSD.
Why am I having this error? And what can be done to resolve it?