One lovely experience I had when trying to remove a failing disk from my array was that the `btrfs device remove` failed with an I/O error - because the device was failing.

I then had to manually delete the file with the I/O error (for which I had to resovle the inode number it barfed into dmesg) and try again - until the next I/O error.

(I'm still not sure if the disk was really failing. I did a full wipe afterwards and a full read to /dev/null and experienced no errors - might have just been the meta-data that was messed up)