Well, I can't tell for sure that it isn't possible, but if I think about it I'd say it isn't. The process of aligning the magnetic vectors on the plater surface needs constant speed, so when power goes away, the platters begin to slow down. Writting at a constantly decreasing speed will lead to vector corruption. I really doubt that any HDD producer would take such a chance. Better lose the unwritten data than corrupting the data that is stored safely already. But the technology advances, so nothing is 100% sure. I will watch this topic, so please, if you finally get a reliable source of information, please share it here.