Reinstall os from scratch?

I had some issues today with my Readynas 102 and I figured out the cause. It would not update and was running poorly so I checked my log files and found the error "Volume: System volume root's usage is 100%. This condition should not occur in normal conditions. Contact technical support." appearing every 10 minutes. After looking around on the forums I figured out it was because of a massive log file recordin ssh access attempts located in /var/log/btmp. I had stupidly port forwarded ssh in my router and my nas must have been under attack for many months. I have now removed the port forward for ssh and I'm hoping that's all that needs to be done. While I was poking around the file system in shell, I wondered how I could reinstall the OS if the web admin page was broken. 

So my question(s) is: If I was to destroy my system by running rm -rf / and deleting everything, how could I reinstall the system? The web admin page wouldnt work so could I somehow boot from a USB to install the default firmware? Is the OS permamantly installed to the NAS or can it be modified? Is there a method to flash firmware without the admin page like a mode that can read firmware from a USB? I understand the ReadyNAS OS is some version of linux (I'm guessing debian?) with Netgear software running on it. How does this all work?