Your Mac user account must be an Administrator.

A flash drive, external hard disk, SD card, or the like, with at least 8GB of capacity.

So you have to quit the installer once it appears in order to keep this file.

After you upgrade to Yosemite, this file is deleted. The 5GB OS X Yosemite installer app that downloads into your Applications folder when you first install it via the Mac App Store.Here's how to make one using Apple's built-in boot drive creation tool. But what if you need to partition the hard drive, wiping the recovery partition? You need a bootable OS X Yosemite installer. Separately, new versions of iWork are also now available as a free download.How do you erase your Mac hard drive and install OS X Yosemite without a startup disk? Normally the answer would be to boot into recovery mode. Users who wish to create a bootable USB installer for OS X Yosemite will want to do so before completing the installation of Yosemite, or, at least make a copy of the OS X Yosemite Installer, which is found in the /Applications/ folder. The download will begin and appear under the “Purchases” tab: Update: Users have reported successfully downloading OS X Yosemite by hitting Command+R to refresh the Yosemite page in the App Store, then try clicking the “Free” button again. This would be a good opportunity to prepare your Mac for the OS X 10.10 update and backing up the computer if you haven’t done so yet. This simply means you should try again later, as the download is likely propagating through Apple servers. Many users are currently experiencing a “This item is temporarily unavailable” error message when trying to download OS X Yosemite. Simply try again, or wait a minute or two. If you are having trouble downloading OS X Yosemite, with either a “null” error, a “We could not complete your request” error, or a timeout, it’s likely due to Apple Servers being overwhelmed by requests. The download weighs approximately 5.1GB and will begin immediately.

To start the download of Yosemite on your Mac, simply open the above link and click on the “Free” button. You can also use the download link below to head directly to OS X Yosemite in the App Store. To download and install the update, head to the  Apple menu and choose “Software Update”, the OS X Yosemite installer is several GB in size and can be found under the “Updates” tab. OS X Yosemite is available as a free download from the Mac App Store. The newest version of OS X runs on all Macs that are capable of running Mavericks, if you haven’t done so yet, you can prepare your Mac for the Yosemite update by following a few simple tasks, including updating your software and backing up the computer before installing the free update.

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