I'd like to erase my HD and I have already booted my mac while holding down opt+command+r. However, the "erase" button for the Macintosh HD is greyed out, just like all other options.
MacBook Pro (13-inch Mid 2012), OS X El Capitan (10.11.6), null
Command R boots you into Recovery Mode. While in Recovery, you should be able to erase your internal drive.
Do you see a window like this? If yes, first try running Disk First Aid.
You should see your drive listed as xxxx then underneath Macintosh HD. Try selecting both the top name to erase. If that fails try selecting Macintosh HD.
xxxx could be Core Storage, name of drive, fusion drive etc.
Erase button is grey on Disk Utility
