I am setting up a NAS server hopefully with a ZFS file system. I have a question I haven't been able to find a straightforward answer to online. I am plaining to have my main OS on a 60gb ssd, I have 4 other 2 TB HHD. When I am installing the OS is it necessary to have all 4 hard drives plugged into SATA? Can I just install the base server and after I am done building plugged in thoes HHDs and configure them? if so how do I go about doing that. And if not what is the best recommended way to do this?
You can certainly do that. After the server is up and running on the single hard drive, install the others using these instructions.
The tl;dr version is that you create a mount point for each drive and update
/etc/fstab to mount the new drives.