I've always thought that only sample libraries can be run from external drives or indeed any volumes other than the startup disk. AFAIK, all plugins (excluding their sound libraries) must be present on the boot drive or they simply won't be seen by DAW hosts.
They don't have to be on the boot drive - one way to do it is to have the user account on a different drive (requires a bit of a workaround, so make sure you try this with a brand new user account so you don't screw anything up).
It might be possible to achieve the same using an alias in the finder (sort of like a "shortcut" in windows, but works a bit differently on a mac)