If you can spin up the HD, read the file into RAM, and spin down the HD, it will take a lot less energy than reading the file slowly over the course of an hour.
Spinning down the hard drive is a pretty bad idea as it doesn't save much power & there is usually a noticeable lag to spinning it back up again. Also most applications & OSes these days expect it to be always on. Your carefully crafted scenario goes up in smoke if your OS likes to log stuff, your browser wants to dump it's cache or your word processor wants to autosave.
