mkfs.btrfs refuses to operate on a block device with mounted partitions, even if doing so is perfectly safe. An example when this happens is when using systemd-repart with it's --image switch to add a root partition to a /usr only image. As a workaround until the issue is fixed, let's hide the information on mounted filesystems from mkfs.btrfs so it doesn't fail and formats the new filesystem as expected.
System and Service Manager
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