A drive failed in my btrfs array that has four encrypted disks:

Label: 'hogwarts' uuid
Total devices 4 FS bytes used 3.13TiB
devid 1 size 3.64TiB used 2.17TiB path /dev/mapper/gryffindor
devid 2 size 3.64TiB used 2.17TiB path /dev/mapper/slytherin
devid 4 size 2.73TiB used 1.24TiB path /dev/mapper/ravenclaw
* Some devices missing

The bad drive didn't even show up in the bios. So I pulled it out and plopped in a new drive to replace it:

# btrfs replace start 3 /dev/mapper/hufflepuff /mnt/hogwarts

Optionally watch it progress for a while:
btrfs replace status /mnt/hogwarts
...
Started on 10.Apr 12:40:05, finished on 10.Apr 15:13:14, 0 write errs, 0 uncorr. read errs

Good as new!
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