kid2/kid3 : new disk situation

So now kid3 only has two remaining disks: 500GB for system(s) and 2TB for btrfs /home

The WD 2TB drive has been moved to kid2 and after twiddling with the BIOS boot order kid2 went online again and happily started a full backup of run12l.

So I have a little bit of time to fiddle with the btrfs again.  Here's the situation (in addition, there's still an unconnected HDD in kid2, don't know what it is).

Systems are on /dev/sdb : 149,1 GiB, 160041885696 octets, 312581808 secteurs
btrfs small part on /dev/sda1 : 596,2 GiB, 640135028736 octets, 1250263728 secteurs
btrfs large part on /dev/sdd : 1,8 TiB, 2000398934016 octets, 3907029168 secteurs
So /dev/sdc is the WD migrated from kid3:

Disque /dev/sdc : 1,8 TiB, 2000398934016 octets, 3907029168 secteurs
Unités : sectors of 1 * 512 = 512 octets
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x000ef301

Périphérique Amorçage     Start        Fin   Secteurs   Size Id Type
/dev/sdc1                  2048  976575284  976573237 465,7G 83 Linux
/dev/sdc2             976576512 3907029167 2930452656   1,4T 8e LVM Linux

So after:
sudo btrfs device add -f /dev/sdc1 /var/backup/urBackup/

I get:
Label: none  uuid: 4881245e-bed5-4526-b9ff-3b5ba39b283d
        Total devices 3 FS bytes used 1.10TiB
        devid    3 size 596.17GiB used 567.03GiB path /dev/sda1
        devid    4 size 1.82TiB used 567.03GiB path /dev/sdd
        devid    5 size 465.67GiB used 0.00B path /dev/sdc1

Then to free the small 640GB drive I can use:
sudo btrfs device remove /dev/sda1 /var/backup/urBackup/