My sister had a HP Media Center Laptop at one time that had the ability to handle two hard drives. Most laptops don't come with that option but you can still add a second hard drive by buying a special hard drive caddy that replaces your DVD drive with an hard drive. A Optical Bay 2nd Hard Drive Caddy.
I bought one awhile ago but never got around to using it till today.
So basically you just slide the drive in making sure it is attached to the SATA plugins inside the caddy. There are four screws to put in the back of the caddy to secure in the hard drive. Four screws and a little scredriver came with the caddy.
Then you will have to remove the DVD drive from your laptop. Usually there is one screw under the laptop that you have to remove first, then the DVD drive should pop just right out. Here's what it looks like on mine.
The only problem I had with my caddy is there is no latch (not sure what to call it) for the screw to go back into. So I got no way to securely keep in my system. Luckily, my laptops like me never travel. This is what I'm referring to from a DVD drive.
So I might have to use The Handy Man's Secret Weapon, Duct Tape, on the edge of my laptop to keep this drive from falling out. Otherwise it works. I can access the drive as a second hard drive.