Geexbox has an ISO that is optimized to run on both USB and SD cards and of course all of them have the option to be installed on a hard drive. This makes them extremely portable, plus there are various ISOs for different systems which are about 150MB apiece.
Installing Geexbox into a hard drive is very simple. Just select the install option from the boot menu and it will automatically take you to a screen where you can select your hard drive. It needs very little involvement and it's done in a matter of minutes.