I am using time machine editor to run the backup once a day at 3:00 am. My mac is set to not sleep when screen is off. However my external drive goes to sleep if not in use for awhile. My understanding was that everything is supposed to work regardless if things are sleeping? My last backup failed. The screen was off but computer was prob awake because i have it set not to sleep if screen is off. But the external drive which is the time machine disk was asleep. Here was my error message... 2/16/14, 3:01:46 AM: Starting manual backup 2/16/14, 3:01:46 AM: Error -35 while resolving alias to backup target 2/16/14, 3:01:46 AM: Backup failed with error 18: The backup disk could not be found. I temporally changed the backup to 6:37am in order to make sure it could still find the disk. Everything was awake and it worked fine. So is time machine supposed to work while the mac or the external drive are sleeping? If not, how to fix? Well either way what do think the issue is?