I use an SSD for my primary, so it's all faster, but I have set up PS to use both my primary and secondary drives. But I know that the default is set to primary drives and that most people don't change it.
I use the built-in SSD for my scratch disc. It basically just houses all my software and I use a thunderbolt external for all my files.
As everyone else has said Janifar, delete stuff. Photoshop is a resource-hungry beast and will temporarily use space on your hard drive when it wants more thinking power, so make sure there's plenty.