Q: How to re-download OSX Yosemite from App Store when already installed

It won't let you download the App Store version because your Mac uses a machine specific version of Yosemite. The App Store version doesn't include the hardware drivers for your model, so it won't let you download that version.

Use the recovery partition; it’ll reload Yosemite. Apple’s Internet Recovery system isn’t available on that MacBook Pro model, and won’t download an OS older than Lion in any case.

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Do this when booted from your older Mac:

Install or Reinstall OS X from Scratch

Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Erase the hard drive:

  1. Select Disk Utility from the main menu and click on the Continue button.

  2. After DU loads select your startup volume (usually Macintosh HD) from the

      left side list. Click on the Erase tab in the DU main window.

  3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on

      the Security button and set the Zero Data option to one-pass. Click on

      the Erase button and wait until the process has completed.

  4. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible

           because it is three times faster than wireless.

This should install the version of OS X that you had installed.

Overwrites the data as opposed to just deleting the disk’s directory. This option is generally only needed when deleting sensitive files.

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