I had done a clean install on my Mac Mini. I put Backblaze back on and wanted to get the backup going again. Uh-oh. The clean install means this is a new machine. My $50 license was for a machine that doesn't exist any more. I sighed a lot at this point, considering a session of working through Customer Service and trying to get things changed.
But wait. Surely in the backup game this is a common occurrence. They must get this all the time. I checked the website and sure enough, they do, and they have specific instructions on how to handle this scenario.
I logged onto the Backblaze website and went to my account Overview. Sure enough, at the bottom of the page are a number of options, including "How do I transfer a license to a different computer?". That was my scenario. They instructed me to switch to the Account page, and delete the old computer. That left me with an unused license. By this time, Backblaze on my Mac Mini was doing a free trial backup so the new computer was showing on my Account page. I applied the unused license to the new one, and it took, and all was well.
So now Backblaze is busy backing up my Drobo again.