OP:
I could be wrong, but I don't think it matters from which drive you boot to disable SIP.
Try this, and get back to us:
1. Power down, all the way off
2. Press the power on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears
3. Do you see one recovery partition, or two?
4. Select one of them (or just the one, if that's all there is), and hit return.
5. You should boot to the recovery partition
6. Go to the Utilities menu and open Terminal
7. In terminal, enter the following:
csrutil disable
Write the above command down on paper. There is a space character between "csrutil" and "disable".
8. Now, quit terminal and reboot to your regular boot drive.
To check to see if SIP is really disabled:
1. Open terminal
2. Type this command:
csrutil status
What do you get?