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/sdy and /dev/sdz, it will write to both sdy and sdz.
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.)