How to re-add existing (missing) BTRFs device without removing it first

Question

I added a new HDD to my BTRFS pool (4th device), all was fine for 20 minutes, then Arch Linux reported it as a read-only file system. I mistakenly rebooted and the system refused to boot (stopped at maintenance menu), so I commented out the BTRFs FSTAB entry and rebooted ok. Now we see

sudo btrfs filesystem show Label: none  uuid: 8dac8c7b-e793-4b32-8085-05acb581c7ea
        Total devices 4 FS bytes used 21.28TiB
        devid    1 size 7.28TiB used 7.12TiB path /dev/sdd
        devid    2 size 7.28TiB used 7.11TiB path /dev/sdc
        devid    3 size 7.28TiB used 7.11TiB path /dev/sdb
        *** Some devices missing

Device 4 has meta data on, so I don't want to REMOVE it and re-add. Is there a way to re-add without removing first? Note, that this is the same drive, if I try to add it into the pool, it asks for a -f to force an overwrite of the drive. Is it possible to do without an overwrite, so it reads the drive and realises it is part of the pool.

Answer 1

I had a similar situation, and what worked was to unmount and re-mount using the dev path of the missing-but-not-really disk:

mount /dev/sdb2 /media/source # (no good)
umount /media/source
mount /dev/sdc2 /media/source # (good)

Hope it helps. BTW btrfs device scan has always shown nothing for me.

Answer 2

I did this:

$ sudo btrfs filesystem show /mnt/btrfs/
Label: none  uuid: 162e1cf7-d4f7-4421-80e4-1b88123ace02
    Total devices 2 FS bytes used 588.05GiB
    devid    1 size 1.24TiB used 590.01GiB path /dev/sdc3
    *** Some devices missing

$ sudo btrfs replace start -B 2 /dev/sdd3 /mnt/btrfs