EDIT: It is definitely not built-in. I had auto-start of my array enabled so it looked like my VM started without the array.
Editing my old post for clarification.
Thx itimpi for explaining what happened!
ORIGINAL (WRONG):
Well, seems like this functionality is already built-in! Sorry for the confusion.
I had a power outage due to a short at an outdoor power outlet.
I was not at home and suddenly couldn‘t connect via VPN anymore. What really happened is that my server did not automatically boot from the USB-stick. The server was just idling and stalled at the boot screen.
After correct rebooting Unraid started array checking which of course shuts down the array. BUT, my pfsense VM was booting up fine!
My VM HDD image is located (fixed) on my cache drive.
Works perfectly! Thx Unraid