You can, but you need to sacrifice one of the HDDs to hold an OS.
Some people want to spend time building and configuring a NAS. Some just want to use one. I've been down both roads, but the first time I discovered that I'd got a failed HDD on a FreeNAS install, and zero alerts because of it, was the moment I thought of switching to a "proper" NAS.
Biting the hand that feeds IT
