i upgraded to 16-gb RAM, but osx only shows 8-gb

A mid-2012 should be able to handle 16GB. Most likely you don't have one of them seated correctly. It can run with just one, though it's not the best way to do things to answer your question.

Just be careful. Review the instructions for installing the RAM in the manual. It's like a car. If you have to force it, you're doing something wrong. If I remember right, you need to spread the clips on the ends a little to get it to pop up, then you can remove it.

http://support.apple.com/manuals/#macbookpro

That model will handle the memory easily...did you install all of the modules yourself? If you misalligned and forced a module you can cause serious damage by removing with more force than you can exert with your fingers.

Also, there have been a number of users having difficulties with Corsair ram, especially their Vengeance series. They are seldom close enoug in spec for an iMac.

If you cannot remove the ram yourself without causing damage, you need to visit an Apple store genius bar and have the techs remove the ram for you.

You would be better off buying ram from Mac specialists like OWC, http://www.macsales.com where they test and guarantee the ram. MacApproved does not really mean anything.

So this solved it? Great!