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Can I download Mountain Lion from the App Store and install it on my machine later? I have a very slow Internet connection at home, but I am traveling with my MacBook now and have temporary access to a faster connection. I'd like to take advantage of the fast connection to download Mountain Lion, but I don't want to install it until I return home and make a backup of the machine first.

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up vote 7 down vote accepted

Yes, the App store downloads an installer for Mountain Lion and stores it in the usual spot - /Applications.

You should only need a network connection after installing to get updates, etc.. and not to run the installer outright.

Be aware that the installation process deletes the installer once you are done installing the OS, so you might make a backup copy of the Install OS X Mountain Lion program before you run it.

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And you can make an install disc if you need to reinstall later or install on another machine. But you have to do it before running the installer for the first time. – SSteve Jan 28 '13 at 17:48
@SSteve totally correct - I would say back up the installer is sufficient for most people since Apple's installer does better checks than extracting the installESD image which just installs. – bmike Jan 28 '13 at 18:04
To add to this, and to save you a google search unless you're already familiar with how it works, it's saved to your Application folder for some reason. – Tony Jan 29 '13 at 15:45
Thanks for the comments @Tony - I've incorporated the various suggestions from you and SSteve into the main answer. – bmike Jan 29 '13 at 16:02
You must be connected to the internet when you do the install. But downloading the installer ahead of time will mean you don't have to use your slow internet for download, just for verification and registration perhaps. – GEdgar Jan 29 '13 at 16:15

There are a few complications to be aware of, and I think Apple is tweaking the download behavior. The answer above only works if you're an Administrator user, you haven't downloaded ML before, and you don't have an old copy of your install file in reach of Spotlight.

Also the download from App Store launches installer, so you have to quit to save it.

The best description of how to do this that I've found is here:

Briefly (I'll do a blog post on this):

  1. You need to do this from an admin account. I think if you use a non-admin account and quit the installer OS X deletes the download.
  2. If you do this from an admin account Lifehacker suggests creating a fresh admin account for the download. I don't know if that's necessary, but I do know that if you quit the installer the installer does a Spotlight search looking for a previous Mountain Lion OS X Installer and then updates the previous installer. So you won't find it in Applications though the Installer icon will show up in Launchpad.
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