http://3cbzkrvakrpetjjppdwzbzqrlkmzatjs7jbyazap5gwutj32gcltjpqd.onion
It quickly filled up the metadata block group, and since the entire SD card had been allocated to different block groups, I got ENOSPC. Deleting a few files and running btrfs balance resolved it; now it allocated 1GB to metadata, which was plenty. I re-ran the dar extract and now everything was fine. See more details on btrfs balance and block groups .