So I just purchased an Iomega Ultra 750G hard drive for my backup with Time Machine. While the computer is on it backs up hourly (but when done dose not unmount the drive grrrr) but my question is should I leave the HD on despite my iMac set to sleep after a certain time? Will this cause any issues on the HD or will it spin down once the mac is sleep? What is best practice here, this is my first time backing up.