sd card reports full when it isn't
This did indeed turn out to be file system corruption.  This question helped me solve it but I don't think it is a duplicate because the accepted answer did not work for me.  However one of the other answers was very close to what I needed.

To fix:
Connected to my Linux PC with adb.
Then issued adb shell mount | grep -i sdcard command.  This told me that my sdcard filesystem was vfat, which is why the accepted answer to the other question didn't work for me. So going off another answer to that question I did a adb shell fsck_msdos -y /dev/block/vold/93:80.  This spat out a bunch of output and created a bunch of files in LOST.DIR on the sdcard which was awesome, because then I could get at them and delete them.  One was 770M which was the bulk of the missing space.

Problem solved.