And to couple with this, while
btrfs device delete /dev/sdx does not write anything of consequence to
/dev/sdx, if it's in a filesystem with
/dev/sdz, it will write to both
In my case, while
sdz was on the nice internal SATA bus,
sdy was also on the flaky enclosure.
Hilarity ensued. (By which I mean, weeping ensued.)