The SD card is vital. It contains the operating system software for the unit.
You can build up the software from scratch following instructions on the Brennan website. It's not really very difficult. I did it just this afternoon, as it's been a long time since I updated. I keep a spare SD card, so that if anything causes the one I'm using to become corrupted, all I have to do is swap them out.
Here are some notes I have to myself:
Download the Etcher program.
Run Etcher. It is simple and straight forward.
Select whatever the most recent SD zip file that's been downloaded.
On Aug.14,2020, I downloaded B2BNov2019.zip This has new Bluetooth capability (B2B) and is upgradeable.
Click on Flash
It takes a few minutes and progress is displayed.
At this point, you may have to setup WiFi again. In my case, the B2 already knew my password and it was quick.
After booting, the WebUI reported version B2B Mar 17 2020.
That could be mislabeled, because B2B wasn't available until April 17.
Now you're ready to do either a Web Upgrade or a SW Upgrade (from USB)
Copy the b2.tar.gz (which has the version of choice) to a USB stick, and plug it into USB C.