Creating a bootable copy of Mountain Lion on a USB flash drive isn't difficult; all you need is the InstallESD.dmg file that you copied to your Desktop on page 2 of this guide (and a flash drive, of course).
Erase and Format the USB Flash Drive
- Insert the USB flash drive into your Mac's USB port.
- Launch Disk Utility, located at /Applications/Utilities.
- In the Disk Utility window that opens, scroll through the list of devices in the left-hand pane and select your USB flash device. It may be listed with multiple volume names. Do not select a volume name; instead, select the top-level name, which is usually the name of the device, such as 16GB SanDisk Ultra.
- Click the Partition tab.
- From the Partition Layout drop-down menu, select 1 Partition.
- Click the Options button.
- Make sure that GUID Partition Table is selected from the list of available partition schemes. Click OK. Warning: All data on the USB flash drive will be deleted.
- Click the Apply button.
- Disk Utility will ask you to confirm that you wish to partition the USB device. Click the Partition button.
The USB device will be erased and partitioned. When that process is complete, the flash drive is now ready for use as a bootable device for OS X Mountain Lion.
Copy the InstallESD.dmg File to the Flash Drive
- Make sure the USB flash device is selected in the device list in Disk Utility. Remember: do not select the volume name; select the device name.
- Click the Restore tab.
- Drag the InstallESD.dmg item from the device list (it will be near the bottom of Disk Utility's device list; you may need to scroll down to find it) to the Source field.
- Drag the USB flash device's volume name from the device list to the Destination field.
- Some versions of Disk Utility may include a box labeled Erase Destination; if yours does, make sure the box is checked.
- Click Restore.
- Disk Utility will ask if you really wish to perform a restore, which erases all information on the destination drive. Click Erase.
- If Disk Utility asks for your administrator password, provide the information and click OK.
Disk Utility will copy the InstallESD.dmg data to the USB flash device. When the copying is complete, you will have a bootable copy of OS X Mountain Lion ready for use.