InstallESD.dmg should be mounted, ideally. This will expose the 4.8GB "Mac OS X Install ESD" partition which is the only bit that needs to be restored to a suitably-sized partition on a storage device with a GUID partition map (i.e. one that is bootable by an Intel-based Mac).
I used an 8GB USB pen drive that has two partitions: one just large enough to hold the Mac OS X Install ESD data (I made it 5.0GB), and the second partition is for arbitrary data (so the pen drive still has a use other than being just an ML installer). I really did this partitioning bit just to clarify the point that all you need is a partition on the destination device
just large enough to hold the contents of the Mac OS X Install ESD partition that is exposed when you mount the InstallESD.dmg image.
I took a short screen grab video to explain this process visually. There's no commentary but hopefully this will be useful to someone.
Note that you do not need to make two partitions on your destination device; but you must have at least one partition large enough to hold Mac OS X Install ESD and the device should be partitioned w/ a GUID map.
https://dl.dropbox.com/s/9il2xov8com2d67/creating-a-ML-USB-installer.mov (24.1MB)
The overall gist of what is done here:
- Open Disk Utility
- From the Contents of the Mountain Lion installer app, drag InstallESD.dmg to the left pane of the Disk Utility
- Right-click the .dmg file and choose Open to "attach" (mount) it. You'll see Mac OS X Install ESD appear in the list
- Plug in your USB pen drive (or whatever device you're making a ML installer on), select it in Disk Utility and choose the Partition tab
- Make a new Mac OS X Extended (journaled) partition on the device that is just large enough to hold the Mac OS X Install ESD; I made mine 5.0GB (the installer data is about 4.75GB) (note here that you can choose to multi-partition, but you don't have to, you minimally need one large enough to hold the installer)
- Hit the Options button and make sure GUID is selected
- Hit Apply and let it partition
- Click Mac OS X Install ESD in the left column, so that it's highlighted
- On the right side, click the Restore tab
- In the left column, click and drag (important: do not select it, just click and drag) the new partition on your device that you just created. Drop it into the Destination field on the right side
- Click Restore, click Erase, enter your credentials and you should be on your way
I think that's everything.
PS: please disregard my gratuitous highlighting of text in the video. I'm a bit ADHD like that..