It will run using the slower interface. Unless it is a phenomenally (sp?) fast drive, you won't notice the difference. Try it and let us know what happens.
Umm, is the external dock connected to the computer via ESATA 2, or Firewire, or USB 2.x, or USB 3.0? I just inferred the first one by your description of a "sata 2 hard drive dock," but I shouldn't assume things.