Back up your El Capitan data
If you created any files while using El Capitan, now's a good time to save them. (The public beta has been out for about a week, so maybe you have just a few files.) Copy the files to an external drive (different from the Yosemite installer drive) or to a cloud storage service like Dropbox.
Say adios to El Capitan
Created a Yosemite boot drive? Check. Saved your files on your El Capitan Mac? Check. Now it's time to erase the El Capitan volume--yes, you have to erase it, you can't simply install Yosemite over it. If you try to, you'll get a message like the one in the screenshot to the above right. You're going to use Disk Utility on the Yosemite installer drive you created.
- Attach the Yosemite installer drive to your Mac, and at startup, hold down the Option key.
- You should eventually see a drive icon labeled Install OS X Yosemite. Click on the arrow below it.
- An OS X Utilities window will appear. Select Disk Utility and click Continue.
- Once the disk Utility app launches, select the Mac's hard drive in the left column (it's the first one listed).
- Click the Erase tab, and in the windows that appears, name your hard drive, select one of the OS X Extended formats.
- Click the Erase button to wipe out everything on the hard drive.
Once Disk Utility is done reformatting, quit the app. This will take you back to the OS X Utilities window. Now, select Install OS X, click Continue, and proceed through the installation process.
Restore your files
Connect up your Time Machine drive and run Migration Assistant (located in /Applications/Utilities). This will walk you through the procedure of restoring your files from a Time Machine backup. If you copied files from El Capitan to a drive or in the cloud, you can bring them into Yosemite.
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