I have two external devices plugged in to my openwrt router but for some reason it is only detecting the first one /dev/sda. I know it is not the devices malfunctioning because I can get them to appear one at at time if I plug them directly into the board via the single usb port. So far I have tested each individual on the pi, my laptop and desktop (pc) and they all work in those cases. The only time they don´t work anymore is via the multi usb hub. Not sure if I haven´t format them correctly or the hub itself is to blame. Will see if plugging the usbs in my laptop or desktop via the hub works.
Ah ok! I guess it does not matter since I am using Squashfs version on Banana Pi R64 which does not have extroot, etc any which is what I would have used the extra space for. I can still use the one detected drive to store a backup of OpenWrt itself.
This is largely unnecessary unless you are doing unusual things with your device.
By default, OpenWrt is designed to work without needing to write files except when changes are made by the user. Therefore, there is no need to have constant backups... simply make a backup when you complete you initial config, and then anytime you make changes (which is pretty infrequent for most users).
The only real reason to have storage for 'backups' would be if you want to save logs to an external device. Log files would obviously be written frequently, so you need a place for that to be stored that will not wear out or fill up storage that is critical for core operation. Keep in mind that storing logs on flash media of any type may eventually wear out the memory, but obviously USB flash sticks and similar can make this a non-issue insofar as they are cheap and easy to get. But do not attempt to write logs to the storage that contains the OpenWrt firmware itself.