machine-config operator is degraded due to "no space left on device" in RHOCP 4

Issue

machine-config operator degraded with below error:

$ oc get co machine-config  -o yaml

 ...
 worker/master: 'pool is degraded because nodes fail with "1 nodes are reporting degraded
      status on sync": "Node xxxxxxxxxxxxx is reporting: \"could
      not write image pull secret data to node filesystem: open /etc/mco/.internal-registry-pull-secret.jsonxxxxxxxxxx:
      no space left on device\""'