Thanks @whiskerp. I was literally minutes away from exchanging my 4Tb Seagate for a 2Tb USB-powerd one, so you may have saved me the trouble.
So I've got Windows 7 booting from the internal SATA drive, using whatever defaul configuration the machine came with. Probably safe to assume it's legacy.
What are the steps for this conversion? Do I have to reformat the SATA drive in some way? What changes (if any) do I need to make in the BIOS? (Sorry for my ignorance -- I'm not familiar with a lot of terminology you used).