I would recommend getting 2 of the OWC Mercury Elite Pro one you can use as your Time Machine backup. The second you could use to store your photos library and any other data you want to store.
NEVER EVER EVER use the same EHD for backup and data storage. Why, simple.... all hard disks eventually fail and often with no warning and if you use the same drive for backup and storage when that drive dies you will have lost your data and it's backup. Don't be penny wise and pound foolish, meaning don't be cheap be smart and get what you need in order to safely use a computer.
Personally I go a little overboard because I have lost data and prefer to keep my internal drive relatively free. I keep 2 backup drives, one for Time Machine another for creating a bootable clone of my drive. I also have a large EHD that I use to store large libraries such as photos, music and movies and so on.