SSD & HDD internal HD on iMac

Hello all,

I have an SSD120 which is the start up HD and an internal 1TB HDD. I use Logic Pro and a LOT of plug ins but I need Logic to read the plug ins from the HDD because ive run outta space on the start up HD. When I down load plugins the option to put them on the HDD is grayed out. How can I make the HDD my go-to for plug ins and get Logic to read them from that HD? thanks in advance for any help.