"btrfs filesystem show" gives me "failed to read" errors

"btrfs filesystem show" gives me "failed to read" errors

My /home is a raid1 btrfs partition spanning two disk. As far as I can tell, it's working fine.
This morning I wanted to check the status of the partition so I entered the btrfs filesystem show command and I only got errors :
$ btrfs filesystem show /dev/sdd1
failed to read /dev/sdb
failed to read /dev/sdb1
failed to read /dev/sdc
failed to read /dev/sdc1
failed to read /dev/sdd
failed to read /dev/sdd1
Btrfs v0.20-rc1
What is the problem?