Is it dangerous to use the -y or -Y flag in hdparm during a write, or when the disk is still mounted, or will hdparm refuse to spin-down, or automatically sync and unmount?
(I am sorry for providing an answer which is not an answer, but a comment - for some reason I can't comment until I have 50 reputation.)
Just FYI: I recently issued (by mistake!) an hdparm -y command to my drive while mounted. I don't believe it was writing at the time, but definitely reading. It did make a "funny", mechanical noise, like a rattling, for a few seconds, but since then it has worked 100 % fine. It doesn't seem to be ruined. Yet.