There are many reasons why you may want to use a USB Flash drive to install Mac OS X Mountain Lion. If you need to install multiple copies on a few computers, or even prefer not to have to download a new copy every time you have to install the Mac OS. And from what I’ve experienced, installing from a USB Flash drive is faster than installing via downloading a new copy from the Mac App Store or even from DVD. Also USB Flash drives don’t scratch, DVDs do.
The steps necessary to create a Flash drive to install Mac OS X Mountain Lion are fairly simple. There are just certain parts that need extra attention however, I will highlight these areas.
Also check out: How to Install OS X Mavericks from USB Flash Drive
Step 1 of 3 – Preparation
Show Package Contents
Firstly you actually have to have the OS X Mountain Lion install app downloaded, you can purchase it from the Mac App Store. Once Mountain Lion has been downloaded, you’ll be able to find it in your Applications folder.
If the downloaded Mountain Lion app automatically pops up after downloading, don’t install it yet, just close the window. What we’re looking for is a DMG file. Go to your Applications folder and locate the “Install OS X Mountain Lion” app. Right click it and select “Show Package Contents”. Once opened there’ll be a folder called “Contents”, open it, and finally open the “SharedSupport” folder. Inside this folder there should be 2 files, one called “InstallESD.dmg” and the other “OSInstall.mpkg”, copy the InstallESD.dmg to the Desktop. This will be the file that we’ll be using to create the bootable disk image onto the USB Flash drive.
Step 2 of 3 – Prepare the USB Flash drive
Format USB drive
To create the bootable disk image, your UBS drive has to have at least 8GB of storage. Nowadays USB Flash drive are really cheap and can be purchased at almost any electronics store (the Flash drive I use is 16GB and only cost $15). Plug it into your Mac, and launch Disk Utility (located in Applications/Utilities). We’re going to format the USB drive, so make sure there isn’t anything on it that you need, as formatting will erase everything on the USB drive.
Select the USB drive that is to be formatted, then select the Erase tab. Under the selection for “Format:” select “Mac OS Extended (Journaled)“, and name it anything you’d like. It would probably would be best however to make it something recognizable like “Mac Install” or “Mountain Lion Installer”. Up to you though. Then simply press the “Erase…” button on the bottom right when done. The USB Flash drive will then start to format; in most cases it takes less than a minute.
If you don’t already have a USB flash drive, or want a USB flash drive that is already compatible with Mac OS X then you can purchase one from the MacMall. It’s a good place to buy Mac friendly stuff. Here’s an affiliate link to a 16GB USB drive that comes already properly formatted for the purpose of installing Mac OS X:
PNY 16GB Attaché 2 USB 2.0 Flash Drive – Black,Blue (P-FD16GATT2-GE) only $11.99.
For more information on formatting check out my other post: How to Format & Repair a Drive with Disk Utility
Step 3 of 3 – Restore Disk Image
Restore Disk Image
Once your USB flash drive has finished formatting, it’s time to “restore” the drive image. In Disk Utility select the partition that was just formatted and click the restore tab. Make sure to select the partition that you’ve created and not the drive itself. To elaborate, as you can see in the screenshot, I’ve selected the partition I named “Mac OS X Install ESD”, not the drive itself. If you select the drive instead of the partition, then it won’t restore the disk image onto your USB flash drive.
There are two input boxes. One labelled “Source:” the other “Destination:”.
For the “Source:” box either drag the “InstallESD.dmg” file in, or click the “Image…” button, navigate to your desktop and select “InstallESD.dmg”. The destination will be the partition you’ve created. Simply drag the partition that’s on the USB drive that you formatted into the “Destination:” box from the panel on the left of Disk Utility. Next simply press the “Restore” button on the bottom right.
This could take a while to complete, let it do its thing. Once the process is done, your USB drive is ready to go.
If you want to use the USB to format a new computer or re-format your current computer, there are 2 ways of getting it installed. One is to go to System Preferences -> Startup Disk -> and select the USB drive. This will load up the Mac OS X Mountain Lion installer as soon as you restart your computer. The other method is to simply hold down the Options key on the keyboard when your Mac is just turning on after being shutdown/restarted.
Once it’s done booting into the installer it’s a simple process of going through the provided setup steps (easy stuff).
Did you know you can run Mac OS X Mountain Lion from a USB Flash drive? Here’s how: Boot from portable USB disk
You can even boot from an old formatted iPod Classic, here’s a guide: How to Boot and Run Mac OS X from an iPod Classic
I tried to copy the install.dmg to the flash drive, but when i click restore it says the resource is busy and it wont work
April 15, 2013 at 5:00 pmI had the same problem and disconnecting and reconnecting didn’t work
April 22, 2013 at 10:03 pmCould you go into more detail? What steps did you take, the more descriptive the better I may be able assist you.
April 23, 2013 at 12:36 amif u just choose the usb drive and not the partition u will get this error
April 25, 2013 at 4:42 pmYou are correct Calle, when restoring the drive image the partition should be selected, not the drive itself. I should have been more clear about this. I’ll update the article.
Thanks.
April 25, 2013 at 11:54 pmcan i update this direct from my operating system Mac OS X 10.5.8
May 19, 2013 at 10:05 amUnfortunately no, OS X v10.6.8 or OS X Lion has to be installed in order to update directly from your operating system.
Here is some more information on Mountain Lion system requirements: http://support.apple.com/kb/HT5444
May 20, 2013 at 12:47 amI have a 32GB usb disk . I wish to have 2 partitions and use one partition to make Mountain Lion USB installer disk. 2nd partition for storage purposes. is it possible? Thanks
May 29, 2013 at 12:27 amYes.
May 29, 2013 at 12:43 amCan I install it from the flash drive without erasing all the data on my laptop? I’m doing this for Mavericks DP 1, but I’m still not sure about that. Thank you.
June 15, 2013 at 2:49 amYeah, it’s not a problem. In disk utility just create another partition on your hard drive, then install Mavericks on that partition.
June 15, 2013 at 2:51 amthanks! Good idea so it doesn’t messes up with my stuff, but is there a way to just install it on top of ML? Like you would do with the final release. I didn’t have this problem with the ML previews. I would just install them normally from the installer, but Mavericks isn’t letting me do that for some reason.
June 15, 2013 at 3:05 amNot to sure if you can install it over ML. And you may not want to use it as your main OS yet, it’s still in its first beta and very buggy. The partition method is currently the best choice.
June 15, 2013 at 3:14 amDid you try this? Same issue here but I don’t want to risk it
August 10, 2013 at 10:29 amthanks for the article! strange though it doesn’t work over here. After the 1. step erasing and creating a partition (extended journaled), the 2. step erases and formats the partitition as (just) exteded and is not recognized as a bootable drive. Any ideas?
June 17, 2013 at 5:42 amMy macbook pro mid 2010 13inch laptop’s hard drive is done. I don’t know why but a question mark folder pops up so I’m buying a WD hdd 2.5-inch 500gb 7500rpm and I want to install ML off a USB. If I buy it on an older mac and follow your instructions should I be able to install it on my new hdd?
June 22, 2013 at 12:34 amYup, you can install it on a new hard drive. Not a problem there.
June 22, 2013 at 12:39 amAfter choosing start up disk as mentioned by you , when I restart my mac I am unable to start mountain lion.please give me solution of this.
July 2, 2013 at 4:48 amAre all your other external devices removed?
July 2, 2013 at 12:05 pmI tried to restore ML on my corsair flash voyager USB 3.0 16GB and halfway through restoring i received an error saying that it unexpectedly shut down so i took out the usb then i tried to plug it back in then i get an error message saying that the flash drive is unreadable. So I plug it into windows and diskpart in command prompt says there is ‘no media’. What the F***? now my windows 8 laptop won’t detect my flash usb anymore.
July 3, 2013 at 11:46 pmIf you formatted the USB drive to Mac Journaled then it will no longer be recognized on Windows. This is just because Mac Journaled and Windows isn’t natively compatible. I’m not to sure what went wrong with during the process. However to fix the USB drive, just format it in Disk Utility again.
Also if you want to use it on Windows again, you’ll have to format it in Windows (NTFS).
July 3, 2013 at 11:53 pm