Q: How to prevent Mountain Lion from going into sleep mode while creating image with disk utility?

Apple Menu>System Preferences>Energy Saver:

Select Computer Sleep = Never.

Are you talking about the screen blanking during the install? if you are that is normal, the machine is not asleep, only the display and tapping the space bar will wake the screen.

i have the exact same problem. "Csound1"'s suggestion works fine for a fully booted system, however the problem that Pekett and I are describing is when booting from a thumb drive, which is a bare bones install system, and does NOT offer the Energy Saver control panel. When trying to run diskutility to do a big disk backup, it halts part way through due to the system going to sleep. This is a serious problem.

Having this exact same issue -- trying to do a big drive backup after booting to a USB stick, but the entire system goes to sleep after 10-15 minutes.  Ridiculous that I have to babysit the mouse and keep it awake to complete a backup.  Nearly an impossible task for a +1TB drive.  Anyone find a solution to this yet?

I did not boot from USB but used Command-R, then launch Disk Utility to create disk image. Same issue that my MBA will go to sleep in 10 minutes. If I hit a key to wake it up, the image creation process will continue. Very frustrating.

Appreciate any suggestion.

i found a somewhat idiotic manual workaround. It's almost embarrassingly simple, but shame on Apple that I had to resort to this:

Find a heavy object (i use one of those big scotch tape dispensers) and wedge it over the keyboard so that it is leaning on the Space key so that the system thinks you are constantly using the keyboard. The system won't go to sleep, and constantly hitting the Space key seems to have no negative impact on the backup.

rex wilsonI used your workaround as well and it really saved my day (another one was already gone). It was very annoying to press keys in the keyboard or move mouse every 10-15 minutes while backing up data, scanning image, etc. with 1TB HDD.

I tied my wired mouse to an oscillating fan while doing a backup. That would slowly move the mouse back and forth over my mouse pad and keep the machine awake during the 8 hours it takes to do a backup.  I'll try putting a stapler on the space bar next time!

I have been having this issue with my new Late 2012 27" iMac - since day one.  It's currently on 10.8.5 and soon to be upgraded to Mavericks.  Disk Utility is not preventing the Mac from sleeping while creating a disk image via the recovery partition and it is necessary to babysit the process to ensure that it runs to completion.  This is very frustrating

I just submitted a bug report to Apple regarding this issue.  The typical Mac user may not ever notice this problem because they would not likely be using disk utility to create system images on a regular basis.  I will report back when I hear somthing from Apple.

~Scott

I have not heard back from Apple on my bug report.  In the meantime I have found an acceptable and easy solution to this issue.  It seems the problem is with disk utility not telling the system "not" to sleep during disk image creation - yet during a Scan Image for Restore - it does keep the system awake.

When you boot into the recovery partition - by default the sleep parameters are going to be set to Display Sleep 10, System Sleep 10 and Disk Sleep 10 (minutes).

Before using disk utility from the recovery menu - click on utilities and open a terminal window.  At the prompt type the following command (no need to elevate privileges):

pmset -a sleep 0 displaysleep 0

Quit terminal - and return to recovery menu - and then open disk utility.  The system and display will stay awake indefinitely.  Until you reboot.

The pmset -a sleep 0 displaysleep 0 - tells power management to disable the system sleep timer and display sleep timer.  These settings are being made in the recovery partition - but unfortunately they will not persist and will be reset back to 10 minutes the next time you boot into the recovery partition.

Hopefully this helps everyone - it is working great for me.

~Scott

Sounds like a nice elegant solution.

The heavy object on the space bar is working for me at the moment. I'd forgotten about this problem until after I'd started the disk image creation process, so couldn't reset the power management parameters.

To way9e0:  You can use the following commands in recovery mode (instead of the heavy object on the spacebar). 

Before using disk utility from the recovery menu - click on utilities and open a terminal window.  At the prompt type the following command (no need to elevate privileges):

pmset -a sleep 0 displaysleep 0

Quit terminal - and return to recovery menu - and then open disk utility.  The system and display will stay awake indefinitely.  Until you reboot.

I think this was resolved in Mavericks 10.9.1 ir 10.9.2 (not sure - haven't had an opportunity to test fully) - but the pmset command above will solve all your problems.

~Scott