Comment 12 for bug 880645

The error as originally reported is due to btrfs converting meta and system data to RAID1 when adding a second device to a single device filesystem and rebalancing. You can then no longer go below 2 devices. Check dmesg or /var/log/syslog as suggested. The remedy is to mconvert to single and remove the device then. Then another mconvert to DUP. Cumbersome indeed, but not a bug.

Most comments from #8 onwards are about different issues and should be dealt with in their own tickets.