Opening an OS X disk image with a launchd agent while the display is sleeping
It is likely your MacBook is asleep when launchd is scheduled to run your disk mount command. launchd does not change its behaviour because of the state of the display.

Your launchd job will run so long as:


the Mac is awake
the launchd job is active
you are logged in (the job ticket is in ~/Library/)


Use pmset's debug logs to find out if your Mac was awake or asleep. The following command will print a log of everything pmset knows:

pmset -g everything


Be sure to check the system.log for messages related to your failed launchd job. If a problem is encountered launchd will log a message.

Applications &gt; Utilities &gt; Console.app &gt; system.log


Mac OS X Energy Saver includes the ability to power on or wake your Mac at a predefined time. You might want to use this ability to wake your Mac a minute before your launchd job is scheduled.

System Preferences.app &gt; Energy Saver &gt; Schedule…