Maximum hard drive thickness

The Late '12 models only has room for two 9.5 mm drives. My home system has two Samsung Spinpoint M9T 2 TB drives (now part of Seagate).

Only the newer Late '14 model can support a single HD that is 12.5 mm. As it has a blade SSD mounted in the lower bay (what you see when you pop the bottom plate off).

The space is fixed on both models so you can't steal the space to give to the other bay to make up the difference. But with that said, if you had access to a 3D printer I'm sure you could make a custom frame to allow you to do something different.