Looks like something corrupted the drive filesystem. I was able fsck repair the drive, and re-apply the image via armbian-imager.
The fact that armbian-imager 'validated' the flashed contents multiple times is where things went sideways. Once i tried to manually delete the contents of the drive, and that failed, it became apparent that the drive had been corrupted. I'm not sure what the 'validation' that armbian-imager does is, though.
