Couldn't unmount disk.

[maj]

Hi. I'm trying to restore OS X Yosemite from an external to my hd. But when I try to repair my hd, it says "Couldn't unmount disk.
Same goes when I try to erase it.
And I can't set my hd as the startup disk. What do I do? Help please.

 

[cradom]

You should have a restore partition on your drive. Reboot and hold down Option until you get a choice of install drives. Select the restore disk.
You cannot erase or repair the drive you booted from.