RN212 - Added second disk to existing

You are confusing "drives" with "volumes".  If what you did was add a second drive with XRAID still enabled, then the NAS used that drive to add RAID1 redundancy to your existing volume, so it remained exactly the same size as your original non-redundant volume.  That's what the synchronization was all about.  That's the expected behavior, though I gather not what you wanted.  Unfortunately, going backwards from that is not something that can be done via the GUI.  The best solution, unless you have experience with the Linux command line, is to back the data up, turn off XRAID, destroy the volume, and then create two new non-redundant volumes (or one large RAID0/JBOD, though I recommend against those configurations).