Please remove inactive volumes in order to use the disk. Disk #1,2 - #25684491

My procedure for fixing:

1. Remove both RAID1 drives.

2. Install drive 1 in up-to-date Linux system (with btrfs 4.0), login as root.

3. "cat /proc/mdstat". Observe device of main volume, /dev/mdXXX (eg. /dev/md127)

4. "btrfs check --repair /dev/mdXXX" (where XXX is according to previous step). May take an hour or more.

5. "mount /dev/mdXXX /mnt"

6. Deleted excess stuff from drive ("cd /mnt", "rm -rf JUNK").

7. "umount /mnt"

8. Reinstall drive 1 into NAS. Turn on. Should now show up as ok but "Degraded" (since only one drive).

9. Shutdown. Reinstall drive 2 into NAS. Turn on. Should now show "Resyncing".

10. Wait. Done!

For the Linux system, I used VirtualBox and an Ubuntu ISO (actually Lubuntu but then had to install btrfs and mdadm), and did the whole thing on Windows with the drive installed an external USB caddy (you'll need to turn on USB3 support in the virtual machine's settings).

Possibly you could do this by SSHing into the device, but the NAS I was working on was a 2-disk and refused to turn SSH on with no available volumes.