S.M.A.R.T. actions need to be blacklisted/disabled on HyperV.
When attempting to use S.M.A.R.T. on HyperV, either from the widget or Diag > S.M.A.R.T. Status, the VM will crash. It appears as though HyperV is disconnecting/removing the disk, as the VM will either show filesystem errors in the GUI or crash/panic in a way that suggests the disk disappeared.
S.M.A.R.T. has no meaning/function in such an environment, but people can still trigger it unintentionally by adding random widgets or maybe restoring a config with the widget from a system that was previously on bare metal.
Not really a bug in pfSense since it's HyperV that's overreacting, but if that is something we can detect it would make for a better user experience.