Cannot delete btrfs pool

I’m trying to delete this pool:

lxc storage show docker
  size: 30GiB
  source: /var/snap/lxd/common/lxd/disks/docker.img
description: ""
name: docker
driver: btrfs
used_by: []
status: Created
- none

The image exists.

But when I try to delete it:

lxc storage delete docker
Error: Failed removing mount path "/var/snap/lxd/common/lxd/storage-pools/docker": Failed removing "/var/snap/lxd/common/lxd/storage-pools/docker/custom": unlinkat /var/snap/lxd/common/lxd/storage-pools/docker/custom/migration.169103354/.migration-send/containers: read-only file system

I suspect it might have something to do with some failed attempts to migrate some volumes and containers from another system.

I don’t need the pool and want it gone.

Any ideas please?

Manage to sort it by creating a loop device for the image, mounting it, then using btrfs to make the filesystem not read only. Then I could delete the pool.