I have one drive with data on it in a zfs pool. I have another drive of the same size that I would like to add as a mirror device to the first one.

In the unraid Main window I don't see any option to do this with the current pool. If I create a new pool with 2 devices then I can choose to mirror the devices. But then I would have to unnassign the drive with data on it from the first pool and add it to this new one. Is that the way to go or would I have to format the drive before doing it?

Of course I could just copy the data off the first device to a temporary backup. Then format the first and second drives into a new zfs mirror pool, and then copy the data back. But as I don't have any spare drives I would like to find a way without having to format the current drive with data on it.

I'm sorry if I am unclear or anything alike as I am a bit new to unraid and don't want to lose anything by accident.

Stop the array and increase the number of drives for the current pool.

When I do that I don't see any option to enable mirroring, unless that comes up when starting the array again (which I am afraid to do since I don't know if it will break stuff). I currently only see the option to mirror when I create a whole new pool with two devices, but then I would have to reassign the drive with the data on it and I don't know if that will break stuff.

You just add a slot and assign a device; don't change anything else, Unraid will automatically mirror the original device after array start