Yup, it spins down. Most of the time just sits there quiet as a clam. (A clam with a little green light.) Only kicks in once an hour briefly to do the automated TM backup. Also, if I know I'm having a day of little work and I don't feel I need the backups, I go into Time Machine preferences and turn it off. Then it's just a question of either running backups manually or remembering to set it back on to automatic when I'm crunching through tasks.