Deleting Mac's main driver

[Dayowas]

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.

 

[Cory Cooper]

Hello and welcome.

The top Macintosh HD (IMG_2206.JPG) is the actual hard drive device itself. The indented second Macintosh HD (IMG_2208.JPG) that has the erase button is the volume partition, which is what you want to erase.

Hope that helps,

C