ReadyNas 3138 Internal sata ports

They are perfectly designed for use in SSD tiering, but not directly supported by the GUI for that.  Quite some time ago, I saw in this forum (but now cannot locate it) where someone did just that by first creating the tiering in a standard bay and then moving the drives to those locations.  It is also possible to simply use them as additional drives by first creating or expanding a volume via SSH.  Once that is done, the OS will recognize the volumes on the Shares pages, but only partially on Volumes.  The volume will appear on the left, but the drives that make it up will not appear on the right.  Tghe OS will support most functions on that volume -- maintenance, backup jobs, standard access, etc. -- but no expansion or similar.

I have a ReadyData 5200 converted to OS6 with a non-Netgear SAS expansion chassis, and that's what I do for the drives in the external chassis.

To get an ideas of what you can do, enter volume_util  --help in SSH.  You can even use mdadm and BTRFS separately instead of Netgear's volume_util, but it's best to stick with the utility unless you need to do something it can't.