External Hard drive not backing up properly with Time Machine

Hi, I recently bought a seagate FreeAgent Pro for backup use with time machine. It backed up perfectly the first time, but it is supposed to back up every hour, and it says it cannot backup, and it has been delayed unless i navigate into the hard drive volume in the Finder. If I don't do this it doesn't backup. I think it is because the drive is going into sleep mode,but I can't find anywhere to change these settings. Anyone have any ideas?

Mike