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Time Machine external drive unmounts when nobody is logged in, backups failing

In tracking down why my TimeMachine backups are returning every hour with "Not starting scheduled Time Machine backup - time machine destination not resolvable." unless I'm logged in, it appears that unless there is an active user on the system, OSX is unmouning my Time Machine drive. Is this normal?


How do I get this system to do the hourly backups when nobody is logged in?

Mac OS X (10.6.8)

Posted on Jul 1, 2011 12:34 PM

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3 replies

Jul 1, 2011 1:29 PM in response to Kappy

Putting drives to sleep is already unchecked, this is not the problem, but thanks for the suggestion anyway.


I believe I have found the issue, it appears that Apple, in their "wisdom" have determined that all external devices should be unmounted when there is no user is logged in. Perhaps this makes sense for some disks, but for Time Machine disks, this is ridiculous in my opinion. Make a bunch of changes and logout, and poof - no backup of your changes until somebody logs in at a future time hours or days later. Sharing your files to another computer? Better log in to the system occasionally to get those changed files backed up, they will not back themselves up you know. Crazy. This should minimally be configurable for TimeMachine, what is the purpose of hourly backups if they don't actually backup hourly. If you're luck enough to have your Time Machine disk internal, then all will work as expected. Too bad for us iMac users.


Luckily I found a 'fix' here: http://systemsboy.com/2008/04/time-machine-after-logout.html


Essentially the following undocumented preference will turn off this behavior:


sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool true

Jun 24, 2014 12:52 PM in response to Omniver

I had the same problem with Time Machine and solved it by enabling "Enable Power Nap" in Energy Saver:


System Preferences...

Energy Saver

Restore Defaults

Check "Enable Power Nap: While sleeping, your Mac can back up using Time Machine and periodically check for new email, calendar, and other iCloud updates."


2013 21.5 iMac

OSX Mavericks

Seagate 1 TB slim USB 3.0 (power is drawn from USB port), formatted to Mac Journaling File System

Time Machine external drive unmounts when nobody is logged in, backups failing

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