If a disk has never been seen by Swarm and it boots into a Swarm chassis, the disk will automatically format on bootup (or insertion in the case of hot plug).
If a disk has been retired from a cluster and needs to be reformatted, you need to access the System Menu on the Swarm storage node via remote terminal or Keyboard/ Monitor.
NOTE: To reformat even a single drive through this interface, the entire chassis will need to be taken offline during this process.
Choose 4. System Control;
Then choose 3. Stop storage processes.
Confirm that choice by choosing Yes.
At that point, you should see Storage Processes: STOPPED
Then choose 2. Disk Volumes
Arrow down to the disk in question and press "f"
The prompt will ask you to type: FORMAT to continue. There is no formatting actually taking place at this point, so do not expect a delay. The disk is being marked for re-format on next boot.
At that point, you can reboot the system under System Control.
The disk will be reformatted as the chassis mounts.