Meanwhile I think this is a more deeper Linux bug, because...

This server has (besides some Seagates) 2 Hitachi (HGST) SATA disks, but
Linux shows 3 of them with 2 with the same serial number:

root@toy02:~# lsblk -o NAME,SIZE,FSTYPE,LABEL,MOUNTPOINT
NAME     SIZE FSTYPE LABEL MOUNTPOINT
sda      1.8T drbd
sdb      1.8T btrfs  data
sdc      1.8T drbd
sdd      1.8T drbd
sde      1.8T drbd
sdf       68G
|-sdf1  14.9G ext4         /
|-sdf2  14.9G ext4
|-sdf3  14.9G swap   swap  [SWAP]
`-sdf4  23.3G ext4   tmp   /tmp
sr0     1024M
drbd0    1.8T btrfs  data
drbd1    1.8T
drbd2    1.8T btrfs  data
drbd3    1.8T btrfs  data

root@toy02:~# hdparm -I /dev/sdb| grep Number:
        Model Number:       HGST HUS724020ALA640
        Serial Number:      PN2134P5G2P2AX

root@toy02:~# hdparm -I /dev/sdd| grep Number:
        Model Number:       HGST HUS724020ALA640
        Serial Number:      PN2134P5G2P2XX

root@toy02:~# hdparm -I /dev/sde| grep Number:
        Model Number:       HGST HUS724020ALA640
        Serial Number:      PN2134P5G2P2AX

This is why btrfs gets confused:

root@toy02:~# btrfs_info
/data :

Data, RAID0: total=108.00GiB, used=107.55GiB
System, RAID1: total=32.00MiB, used=16.00KiB
Metadata, RAID1: total=1.00GiB, used=139.59MiB

Label: data  uuid: 411af13f-6cae-4f03-99dc-5941acb3135b
        Total devices 3 FS bytes used 107.68GiB
        devid    1 size 1.82TiB used 37.03GiB path /dev/drbd0
        devid    3 size 1.82TiB used 36.03GiB path /dev/drbd2
        devid    4 size 1.82TiB used 37.00GiB path /dev/sdb

/dev/sdb is WRONG, it should be /dev/drbd3, because I have expanded the
btrfs filesystem with:

root@toy02:~# btrfs device add -f /dev/drbd3 /data


More info:

root@toy02:~# lf drbd /dev
dRWX       - 2015-08-10 14:17 /dev/drbd
lRWX       - 2015-08-10 14:18 /dev/block/147:0 -> ../drbd0
lRWX       - 2015-08-11 14:45 /dev/block/147:1 -> ../drbd1
lRWX       - 2015-08-11 14:00 /dev/block/147:2 -> ../drbd2
lRWX       - 2015-08-11 14:00 /dev/block/147:3 -> ../drbd3
lRWX       - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../../../drbd3
lRWX       - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../../../drbd2
lRWX       - 2015-08-11 14:45 /dev/drbd/by-disk/disk/by-id/ata-ST2000DM001-1CH164_W241L86K -> ../../../../drbd1
lRWX       - 2015-08-10 14:18 /dev/drbd/by-disk/disk/by-id/ata-ST2000DM001-1CH164_W241L88L -> ../../../../drbd0
lRWX       - 2015-08-11 14:00 /dev/drbd/by-res/d2 -> ../../drbd2
lRWX       - 2015-08-11 14:00 /dev/drbd/by-res/d3 -> ../../drbd3
lRWX       - 2015-08-10 14:18 /dev/drbd/by-res/data/0 -> ../../../drbd0
lRWX       - 2015-08-11 14:45 /dev/drbd/by-res/data/1 -> ../../../drbd1
bRW- 147,000 2015-08-10 14:18 /dev/drbd0
bRW- 147,001 2015-08-11 14:45 /dev/drbd1
bRW- 147,002 2015-08-11 14:00 /dev/drbd2
bRW- 147,003 2015-08-11 14:00 /dev/drbd3

root@toy02:~# lf HGST /dev
lRWX - 2015-08-11 16:41 /dev/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../sdb
lRWX - 2015-08-10 16:45 /dev/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../sdd
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../../../drbd3
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../../../drbd2

root@toy02:~# lf sde /dev
lRWX       - 2015-08-10 16:45 /dev/block/8:64 -> ../sde
lRWX       - 2015-08-10 16:45 /dev/disk/by-path/pci-0000:08:00.0-scsi-0:1:2:0 -> ../../sde
bRW- 008,064 2015-08-10 16:45 /dev/sde