it's maybe better to connect the hdd to usb and leave the fw-bus for the FF800. Pretty much any SSD will do for Logic, depending on your use of samples you might want to get a fast one - not just something that saves you battery and heat. As they say, size doesn't matter, but somehow we kinda know that bigger is still better because you won't have to copy to the old drive so often. Use the external drive as a backup and the SSD as scratch and system drive.