Sorry for how vague my question was.
Clarification: This disk has on it the OS from my laptop. Thus, it has already been formatted, of course. It was working fine, but I needed a larger drive in the laptop. I still want to get some data off of it, so I thought I could just add the thing to my desktop.
In "disk management," I can see the problem is that the disk is not initialized. I attempted to initialize, using the MBR style. Sadly, this attempt failed due to "an I/O error."