Disk won't unmount

Hey guys,

I am trying to reformat my Macbook's hard drive as i am giving it to my mum as i have a new macbook and im trying to reinstall Mac OS X Lion on it. (The original firmware that came with it), hiowever there seems to be problem when i tried to unmount it through disk utility - it says it cant be unmounted.

Currently I am running Mac OS X El Capitan on it, but no matter what i do in Disk Utility in the boot up USB for Lion it just wont unmount it, I tried to repair and verify the...

Disk won't unmount