Replacing the drive with an SSD

I've never had a 2120, so I can't confirm if there are additional SATA ports inside.  But I can tell you that the GUI only sees the ports that Netgear advertises the unit to have.  If you put a drive on a "hidden" port, the GUI will not give you an opportunity to do anything with it.  But the core Linux system does see it, so via SSH it's possible to create a volume and then make it visible to the OS.  I have done that on an RN4200V2 running OS6, but only as a test.  I have also heard one can temporarily put one of the main drives on one of the hidden ports, put the new drive in the now vacated port, create the volume, and then swap the drives.  But I have not tried that and I don't currently have a sandbox system with that kind of configuration where I can try.

These methods work because the volumes are mounted by the core Linux system, which sees the "hidden" drives, based on what you do via SSH and the GUI sees them once it's launched.  The GUI essentially sees the volumes, but not the drives on which they exist (if that makes any sense).  But while the volume on the hidden drive becomes available, I don't know which GUI functions will work with them.  Certainly adding or changing a drive in a volume won't work because the GUI will not see the new/replaced drive.  I know you can add/delete/modify shares on them just fine.  I suspect most maintenance tasks will work as well, the drive test maybe being an exception, but I never tested them.