If you are using device mapper (instead of loopback), /var/lib/docker contains metadata informing docker about the contents of the device mapper storage area. If you delete /var/lib/docker, that metadata is lost. In this case, docker detects that the thin pool has data, but docker is unable to make use of that information. The only solution is to delete the thin pool and recreate it so that both the thin pool and the metadata in /var/lib/docker will be empty.
Red Hat Enterprise Linux 7
RHEL Atomic Host 7
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