Couldn't unmount disk.

maj


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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.
 
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Your Mac's Specs
21in. iMac 10.11 --- HP Linux Mint 18
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.
 

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