I use a USB drive attached to my Airport Extreme as my Time Machine. I use a program called Time Machine Editor to set my backups to 3am daily (I don't really need every hour). I'm finding that my backups aren't happening at 3am when I put my rMBP to sleep. The backups fire right up when I wake it from sleep. The USB drive is first mounted, then unmounted when the backup is finished. Is this normal behavior?