RCA Pro12 Missing Storage and USB Settings Menu - Marsmallow

Hi There.

Just wondering if anyone else out there has encountered a similar problem,

I recently purchased a RCA Pro12 Tablet on ebay.

When I received it I noticed a large sticker underneath reading "Demo Unit-Not all features might be fully activated" or something along those lines

Everything seemed to be working fine though, that is, until I decided to install a SD card into the unit for some added storage space.

It does not want to "see" any sd cards I insert into it, but even more than that, when I go to it's settings menu, I cannot find an option for "Storage and USB" anywhere under the settings menu???

(It does however recognise usb sticks plugged into it on it's usb port)

I have googled far and wide, but so far not come across a similar problem on the RCA tablets (or Android 6) out there, so I must admit, i am now completely stumped!

Any ideas as to what to try to get it to recognise sd cards, or to let the "Storage and USb" options appear again under it's settings menu?

Oh yes, it is running android 6. (Marshmallow)

Many Thanks,

Gary