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I have a an external hard drive attached via USB to my 13 inch Mac Book Air 2011 running 10.7.4. I have Time Machine setup to backup to this drive exclusively; this hard drive serves no other purpose.

My problem is that every time I put my laptop to sleep or logout of my user and then wakeup/log back in, the hard drive icon changes from the Time Machine backup drive icon to the regular USB hard drive icon. I've noticed too that my scheduled backups have been failing:

6/20/12 5:56:06 AM: Starting standard backup
6/20/12 5:56:06 AM: Error -35 while resolving alias to backup target
6/20/12 5:56:16 AM: Backup failed with error: 19

The advice from the interwebs indicates that my hard drive needs to be in 'Time Machine mode' in order for the backup to work correctly. Up till now I've been manually initiating the backup. The other piece of my setup is that I'm using TimeMachineEditor since I don't need hourly backups; daily backups are just fine.

How do I configure Time Machine so that I my scheduled backup will work with this external drive?

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That's not normal behavior. Your drive should be available as a Time Machine volume all the time. So all you can do now is manually start Time Machine and it'll work? –  slhck Jun 24 '12 at 12:55
    
@slhck Yes: when it's got the standard USB icon for the drive on my desktop (instead of the Time Machine icon) I can start the backup manually and it will work. I'm sort of convinced that Time Machine is losing track of the drive after I sleep or logout. –  Avery Chan Jun 24 '12 at 23:37

1 Answer 1

Avery,

I'm not sure if you've resolved this yet or not, but I thought I'd add something.

Time Machine does not backup hourly, it backs up when there is something to be backed up. If you don't use the computer for 5 hours, nothing will be backed up during that time - there is nothing to back up! But, while you are using it - downloading email, creating/editing documents, sync'ing photos, installing apps, etc. - you are changing the data on the system and Time Machine recognizes that and backs it up "soon." I've seen cases where I create a new file and within 5 minutes it's backed up.

So you should not need to back up manually, especially as it goes against one of the major selling points for Time Machine (Turn it on and forget it!).

I have found that occasionally, Time Machine "loses" the backup volume. So, I remove the backup disk and then re-select it. That has worked before.

In general, the backup volume should appear/disappear when it's being actively used or it's not currently in use. This is presumably to prevent users from accidentally using it as a normal backup device.

Other considerations: what version of OS are you currently using? Have you upgraded to Mountain Lion? Have you considered a Time Capsule (nice because you don't have to constantly plug in your external drive and use a precious USB port).

If these suggestions don't work, I would consider formatting the disk and restarting the Time Machine configuration.

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