>I think an external one may be easier to install.
An external hard disk is DEFINATELY easier to install, as all you need to do is plug it into a USB port.
To fit an internal one to a desktop PC you need to take the cover off and get invloved with cables etc. For a laptop it is impossible to fit a second hard drive except on a few laptops that do take 2 hard drives.
Note that an external harddrive is NOT any more safe or secure than a "normal" hard drive (perhpas even less so as it can be dropped or lost) so do make sure you back up important files to CD or DVD.