Cannot unmount disk0 resource busy
A couple of thing are not making sense in your description. When you boot to Recovery HD, you should also have about 15 lines of df output with devices running from disk0 to disk12 or more (it varies between OS X 10.8 and 10.11).

In your example, if you want to wipe all contents on what's normally named Macintosh HD and in your case is disk0s2/Macintosh Hard Drive - you could issue the command:

diskutil eraseVolume JHFS+ Macintosh\ HD disk0s2


The \ is needed to escape the space between "Macintosh HD" - quotes would work as well if you prefer that. I would just use Disk Utility to erase the volume and rename it Macintosh HD since that is far easier, there in the main menu of items, and doesn't need any care with spaces, quotes or messing up the disk0s2 and erasing the wrong area of the drive.

The reinstallation problem might be solved by erasing the storage, but that's probably a better stand alone question.


https://support.apple.com/en-us/HT204904


The above article walks you through how to use Recovery HD to erase and start over if that's your eventual goal.