I have a mac mini early 2009 model with a 120GB hard drive but need more space for photo editing. I was wondering how a external hard drive works? Is it something like a 2 hard drive raid set up or would I have to transfer my stuff to the mac mini internal hard drive to edit them then back to the external to store the edited versions? I know this is a beginner question but I have never had to look into external hard drives before
Connecting an external drive -- regardless of whether the computer is running Windows, OS X or Linux -- is like connecting any other drive. Just the physical location of the device is outside the enclosure instead of inside the enclosure.
You can store all your photos on the external drive. You can store them on the internal drive. You can move stuff back and forth... but that's going to create work for you. But that's no different than if you had scratch folders on the same disk as part of your workflow, or two disks located inside the computer's enclosure.
For example, I have a 500GB Momentus XT inside the Mini. I use it for the OS, apps, and my profile data.
However I moved my iTunes library to a 1.5TB external disk hooked up via FireWire800. The idea being that my iTunes library is large. When I launch iTunes, it just works, because I moved my iTunes library there. I also keep VMWare Virtual Machines on the external drive. However I do create some additional virtual disks (for SQL Server TempDB) on the internal drive for better performance (dedicated drive for TempDB, external disk for the rest of the apps I work on).
Again. They're just hard disks. The only difference is their physical location. They work the same as any other hard disks. If you move stuff, you may need to reconfigure applications. This is no different than, say, moving your iTunes library to some new location on the same disk it was currently on.