On a fresh hard disk, I created a GPT partition table with three partitions:
/dev/sdd1 256M for /boot /dev/sdd2 8G for swap /dev/sdd3 ~1T for /
Then, I accidently formatted the raw device /dev/sdd with BtrFS. I thought: no problem - fresh disk anyway. I recreated the partition table, prepared the swap space and now formatted /dev/sdd3 with BtrFS. The boot partition is not yet in use. When BtrFS proofs stable for me I will make that disk my primary boot disk and pool my former LVM volumes into the BtrFS pool - but that is not of matter now. I rsync'ed all my data partitions to subvolumes on the BtrFS partition, configured the kernel and grub and rebooted. All is fine, working, and: Wow, it's faster.
But everytime BtrFS progs are scanning for BtrFS volumes they still see the /dev/sdd volume which I accidently formatted before I redid partitioning. How do I fix it? It seems not to hurt now, but I don't want any utility try to fiddle around with /dev/sdd by accident just because it sees BtrFS there...