2nd Hard Drive for my M18x. Which should I go for?

Hi guys, my first post to this forum and of course it's a request for help!

My M18x has a single hard drive, a 750Gig. I'd like to fit a 2nd hard drive of equal or greater capacity just for data storage. I'm not really a gamer so ultimate speed isn't a factor.

I'd be grateful if someone could suggest a make/model I should consider?

Many thanks

John