How to unload a disk in mac os x Mavericks so that you can partition? (failed to unload disk)
Try booting form external disk:

Create it first with your OS X version.

Attach the USB **boot** drive to the Mac and reboot
Hold down the OPTION key during boot, then select the attached boot drive (typically has an orange icon at the boot menu)
At the boot menu, choose “Disk Utility” (if using an Installer disk, pull down the “Utilities” menu to access Disk Utility)
Go to “First Aid” and verify the disk, then repair if needed
--&gt; Now perform the original task that threw the “Couldn’t Unmount” error