Time Machine basics. Specifically Sleeping Hard Drive?

[PicnicTutorials]

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?

 

[Macman45]

There is a setting for power nap which allows TM backups to run waking the disk if it's asleep. I haven't tried using it myself as if I do miss a backup my Mac just backs up ad soon as I wake it up again I have it set like this to prevent the well known random wake up issues

 

[PicnicTutorials]

There is a setting for power nap which allows TM backups to run waking the disk if it's asleep. I haven't tried using it myself as if I do miss a backup my Mac just backs up ad soon as I wake it up again I have it set like this to prevent the well known random wake up issues

Ya I have enable power nap set

 

[Macman45]

Ya I have enable power nap set

That's the issue then....I have mine set to off.

 

[PicnicTutorials]

That's the issue then....I have mine set to off.

It says if enabled power nap... while sleeping your mac can back up using time machine. So not sure if thats it. It worked this morning though. Not sure why it didn't work the other day. Time will tell i guess...

 

[Macman45]

It says if enabled power nap... while sleeping your mac can back up using time machine. So not sure if thats it. It worked this morning though. Not sure why it didn't work the other day. Time will tell i guess...

As long as it isn't waking up randomly then I'd go with it...I can tell if mine wakes at night as I can see it light up the lounge....it's been an issue for a shrike now, but mine seems to behave now. Hope yours does too.