Hi, I have a RT-N66U running last Merlin 380.70 and a spare one with the fork (Asuswrt-Merlin 374.43 LTS (V43E6)).
Can I copy the config from one to the other, so I have a drop-in-replacement if the first is broken?
If by "copy" you mean save a configuration file and then load that file into another router, the answer is "no".
If by "copy" you mean enter all the same information manually, then the answer is "yes". Similarly, you can "copy" and "paste" from one browser session to another to configure the router
I know you meant save/restore a config file based on the answer above, but there is a distinction (for anyone else reading this). You can essentially "copy" all the settings (manually) into another router and have it ready to go. You can even use the same IP address, as long as both routers are not active on the same network at one time. You can perform the setup using another IP address, and leave it that way until you "need" it, then simply change the IP, or configure it all by wiring it directly to a PC in essence creating a small network.