btrfs WTFs

[amarao_san]

# btrfs device add /dev/sdp1 /media/JBOD
ERROR: error adding the device '/dev/sdp1' - Invalid argument

Reason: [73866.037363] btrfs: dev add/delete/balance/replace/resize operation in progress (dmesg)

How to fix: btrfs balance cancel /path/to/btrfs