I do get/understand that but does that mean i can’t change it after i have the disks added?
I have 4 disks installed already albeit different sizes. Are you saying that i need to delete a disk so that i can then change another already installed disk to a different Raid type?
You need more than a single disk in a pool in order to change it’s raid from single to raid1 given raid1’s minimum disk count requirement of 2. It needs at least 2 disks in the pool so that it can ensure the data is stored on at least 2 ‘different’ devices simultaneously.
I.e you ‘pool’ disks together and then sub divide their combined storage by way of shares (or subvolumes in btrfs speak).
Could you not add the presumably unassigned Empty 298 GB disk to the pool you want to change to raid1 such that the pool meets the minimum requirement to convert in place to raid1 (from single).