Any external hard drive would do, but some great brands are LaCie (prob the best but pretty expensive), Seagate, Western Digital (the brand I have). Now are you only looking to backup photos? You should really be backing up all you info on a regular basis via Time Machine or a cloning program such as CarbonCopyCloner. If you wish to back up all your data (which I would recommend), look for an external HDD about 1 1/2 times the size of your internal HDD's storage capacity. You can get great deals from places such as newegg.com and other online vendors. Look for external HDDs that come with a good 3-5 yr warranty.
Message was edited by: shldr2thewheel--oh and when you get it, plug it into your usb port, open disk utility in the applications folder, erase it, and format it to Mac OS Extended (Journaled) --(unless you are also using Windows on your mac)