Adding new drive to readynas Ultra 2

X-RAID2 will not let you add a second volume. It is designed to give you a single expandable volume. In your specific case, inserting the 3 TB drive would result in adding redundancy and would leave 1 TB of your new disk unused. You would still have 2 TB of storage.

Getting the configuration you want is possible but time consuming.

You would first need to do a full backup of your folders and save your configuration to a file on your PC. Then do a factory default procedure (not installing the second disk yet). This will wipe your data, and also reset your full configuration back to initial installation.

Within the first 10 minutes of starting the factory default, you use RAIDar to set up the NAS to Flex-Raid.

After the factory default completes you
(a) first reinstall your add-ons
(b) Restore your configuration from the configuration file you saved previously.

The order matters.

Then
(c) Install the second disk, and create the new volume from Frontview
(d) delete the shares you want to shift to the new volume from Volume C, and re-create them on Volume D (being careful to maintain their settings and permissions).

Finally, re-load the data into the shares from your data backup.