add important note for manually adding a device

Author: tmalove

modules/move-etcd-different-disk.adoc

@@ -25,6 +25,7 @@ NVMe:: `/dev/nvme*[0-9]\*n*`
 
 .Prerequisites
 
+* You have a backup of your cluster's etcd data.
 * You have installed the {oc-first}.
 * You have access to the cluster with `cluster-admin` privileges.
 * Add additional disks before uploading the machine configuration.
@@ -35,6 +36,11 @@ NVMe:: `/dev/nvme*[0-9]\*n*`
 This procedure does not move parts of the root file system, such as `/var/`, to another disk or partition on an installed node.
 ====
 
+[IMPORTANT]
+====
+This procedure is not supported when using control plane machine sets.
+====
+
 .Procedure
 
 . Attach the new disk to the cluster and verify that the disk is detected in the node by running the `lsblk` command in a debug shell: