After moving the root file system to an external disk with btrfs replace, samba does not show the external disk anymore, but it still says the total size available it the one of the SD card.
After adding a remove backup location, samba considers it as another disk on which the home, group and public shares can be activated. In a shell, the remote backup location is mounted on /media/someID
… but I had not resized the file system to the maximum space available. After running:
sudo btrfs filesystem resize max /
Samba now says the size available is the total size of /dev/sda1, so the 1. above disappeared.
This is still the same. On the remote location, /var/log/auth.log shows that the freedombox is closing and reopening a session every hour. I wonder why.