I have a Netgear R-7800 router with Openwrt. Because overlay was filling up, I configured an USB stick as extroot which works great.
But what happens in case I need to restore a backup?
USB has not been configured so I can't write to that device.
Is there a solution perhaps to make a complete backup when using an USB as extroot memory?
There is no solution for your problem. The only thing you can do is to backup your configuration files, setup extroot and installing your applications again after an upgrade. Extroot is more work then standard installation.