If you have a laptop with a cd/dvd drive in it and only have 1 slot for your drive. You could remove the cd drive and use an HDD caddy in the place of the cd drive, it's a special casing for a 2.5" drive or SSD that can connect with the port where the cd drive connects to.
For my laptop, I bought a crucial mx200 SSD and put it in the place of the HDD. I removed the cd drive and got an HDD caddy. This HDD caddy is located where the cd drive originally resided and holds the HDD, it works perfectly.