best pratice would be a separate drive WD Purple for NVR only and the other drive with WD Red for data.
It depends on your NAS if you can setup the NVR solution in your NAS to write direct onto the WD Purple.
My NAS currently write on WD Green. But the storage become low.
In terms of money, I thought to buy the WD Purple (replace the WD Green) to fit the cameras on my NAS instead of getting new NAS dedicate to the cameras…