Changing MacOS's IP Address While Booted Off Another Drive My server lives in a data center, 1,000 miles away.
This Mac has two hard drives, a primary and a backup. Both are bootable and have the same network settings, so I can switch back and forth between them, remotely. (Screen sharing, System Prefs, Startup Disk).

I just put a new server in service today. Before boxing it up, I use a keyboard and mouse to change the server's IPs from my LAN IPs, to my public IPs. Server went in to service and all was good. Until I rebooted. 

I forgot to change the backup drive's network settings. So when I boot off of the backup drive, the computer is live on the internet with an IP of 172.16.0.7. No bueno. 

I was able to get the server booted back up off of the primary drive, so I'm in service for now. But I have no physical access to this machine. What I need is:

A way to change the IP address of both ethernet interfaces, on the Backup hard drive, while I am booted up off of the Primary hard drive. If I'm lucky, there's a .plist somewhere I can edit and life will be good. If I'm unlucky, Apple will have some truly insane way of storing IP settings, and it won't be possible to do this at all. 

Note: This is an El Capitan install.
Network configuration information is stored in

/Library/Preferences/SystemConfiguration/preferences.plist


To change the manual IP address assignment, change the string value at

NetworkServices:GUID:IPv4:Addresses:0

To find the right GUID, see the UserDefinedName value for each service.

If ConfigMethod is set to DCHP, the Addresses array is ignored. See the screenshot below for an example of DCHP configuration (Wi-Fi) vs manual (LAN).