Conjectured that there was a setting changed in the registry. There I was able to pull out my secret weapon, which was that I hadn't made all that many changes to Windows since I copied the files from my old drive to the new one, so I was able to export and diff the two settings. There were a couple of differences, but changing them didn't fix things. Replacing the new registry with the old also didn't. Doing a recursive diff on the two Windows' didn't highlight anything.
Windows running from the old disk can hibernate.
So there isn't some PROM setting which needs to be changed. Should be identical code. But still Windows doesn't hibernate from that drive. A friend of mine works for Microsoft. His divining agents were no help.