Mac: You can re-download and install your Mountain Lion update on all the Macs you own, but that won't work if you have different Apple IDs on those machines. However, with this workaround you can reinstall from the same copy as many times as you need to.
We've already detailed making an external installer and using that (which will save on bandwidth and time), but there's also a simple workaround to download Mountain Lion and install it across different IDs. Just follow these steps:
- Open System Preferences on the Mac you want to update, then go to the Users & Groups section.
- Press the + button to add a new user, and create an administrator account. (Don't worry, you can delete it later.)
- Switch users and sign into this new account.
- Open the Mac App Store and sign in with the Apple ID you used to purchase Mountain Lion.
- Download Mountain Lion and install it.
- Delete the new user account.
That's all there is to it. While you could technically use this technique to install Mountain Lion on the Macs of everyone you know, you shouldn't go that far. We're posting this tip to give you a way to install Mountain Lion on household Macs that may be in use by various people who don't share the same Apple ID.
$20.99 is a negligible price to pay for a solid upgrade. That said, you are supposed to be able to install it on all your Macs. If you can't because of varied Apple IDs, use this method to get around the problem.