Fair enough, this is a little confusing to me still. At first I had just regular HDDs in the NAS, but the HDD noise was driving me insane and everything was slow so I started with just NVME drives in the NAS and added HDDs later, now all the system stuff is much faster including boot so everything gets read from NVME clearly, so when does the OS get even written to newly introduced HDDs and is it ever used unless there’s issues with NVME?
That happens as soon as you assign usage to a drive (Storage/tier/cache), that’s when md9 and md13 (+swap but HDD and SSD create different swap groups) get set up.