...or we could put TWO into a conventional laptop, in a mirrored RAID [or functionally equivalent] configuration, in order to eliminate the HDD as a single point of failure.

Watts/Gigabyte is a very silly and not usable unit. I can't choose to use only 5GB of the drive to save power. :)
For anyone too lazy to get up a calculator they draw 0.448 Watts, and its the same for the single and dual platter designs.

In Some Industrial Rag Picker know as theINQ, had 32 gb SSD Card previewed last week, For Only $45. Its Notebook/hand held- dweller & just might save Your Nags Some Real Cooking. 
Also I believe Butterfly effect might be greatly Reduced.
von Drashek

Double paragraphs describing the platters. ;)

Cheers,
John