Please help required to upgrade from Snow Leopard to Mountain Lion.

Hello Community!

I have a iMac model 11,3. With 27" screen, Intel Core i7 2.93 Ghz, 12 GB ram, 1 TB hd.

My only internal disk is "Macintosh HD".

It's now running Snow Leopard fully updated to the latest.

I bought and downloaded Mountain Lion from the App Store.

But it refuses to upgrade saying "This disk cannot be used to start up your computer".

This seems so stupid because its starts the computer with Snow Leopard each time.

So please, anyone can tell me what is happening and how to solve this ?

Thanks in advance.