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.
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.