Hello from the future. I ran into a similar issue: Corrupting the firmware of the CA while updating it. In my case, the firmware update failed at the 1-2% mark and the CA showed a blank screen and did not appear to boot. Further attempts to write to the CA also failed, never getting past 3-4%.
The issue turned out to be that I had the Phaserunner programming tool also running in the background. When I closed it, I was able to successfully flash the CA's firmware, and all reads/writes since then have succeeded.
I'm on Linux using the Grin-built USB programming cable.