Fixing a degraded RAID

[Michael Welsh Duggan <mwd@md5i.com>]

I had a disk in my RAID0 die:

  root@maru2:~# /usr/local/src/btrfs-progs/btrfs fi show 8c530f6f-7592-4d57-854d-1fae33ae7cb6
  Label: none  uuid: 8c530f6f-7592-4d57-854d-1fae33ae7cb6
          Total devices 3 FS bytes used 329.66GiB
          devid    1 size 1.79TiB used 357.03GiB path /dev/sdd1
          devid    3 size 931.51GiB used 178.00GiB path /dev/sdf
          *** Some devices missing
  
  Btrfs v3.12-43-gc2081e2-dirty

So, I try to mount it with -o degraded.  That fails, implying that a
degraded mount needs to be read-only.  So I successfully mount it read
only.  Then I want to add a new disk to replace the missing one:

  root@maru2:~# /usr/local/src/btrfs-progs/btrfs dev add /dev/sdg /mnt/
  ERROR: error adding the device '/dev/sdg' - Read-only file system

So I can't add a disk if it's mounted read-only, and can't mount it
read-write?  What am I missing?  What's my way around this?

-- 
Michael Welsh Duggan
(md5i@md5i.com)