1. Seeing that the system drive is the only possible destination is perfectly normal since ARIA is a system component which needs to be installed there.

2. When you arrive on the Custom Install page, notice the little arrows next to the Location field of the Garritan Personal Orchestra package. This means you can change the location for that specific package included in the installer.

3. If you click on the Location field, you will see a menu appear, choosing Other... will give you a Select Location dialog which you can use the select ANY external hard drive or secondary partition available on your computer.