I found out that a drive that used to be part of a raid system that is mounted and running without it, btrfs apparently decides that the drive is part of the mounted raidset and in use. As a result, I had to eventually dd 0's over it, btrfs device scan, and finally I was able to use it again. mailing list thread with more details: https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg32420.html btrfs should probably improve its check to see that the drive is not really used and let me format it.