Hello, everyone! I want to downgrade from MacOS Siera to OS X Mavericks. I have backuped my Mac using Time Machine and I made USB-OS X Installer Installer. I partitioned external hard drive into two parts: the one I use to backup Mac and the other for creating USB-OS X Mavericks Installer. The thing is that when I reload my Mac with Alt/Option, enter USB Installer and open disk utility I can't delete the Main Driver, there isn't just the button to erase it, but the volume("Macintosh HD") has this button. So, how can I solve this problem? (Here's the zip file of some screenshots) Thanks in advance.