Upgrade to El Capitan on dual hard drive machine.

I have a SSD in my main hard drive port and my original HDD now fitted in the optical drive. The two have been working fine together on Yosemite but now that I have upgraded to El Capitan I can't access any of the files on the HDD. I can still see them but they are non-selectable. I assume the HDD will need to be re-formatted or is there another, easier, way around it?

Thanks!