JBOD over multiple disks

Can you post a screenshot?

What do you see if you click on the SATA 2TB disk on the right? Are you then able to click the New Volume button at the right under the X-RAID button?

This pic is with the right SATA 2 TB disk clicked. If I unclick it the 3 buttons in the blue box go away. New Volume will let me add a new volume as normal.

I would prefer a single volume, so if this is now possible I'll do it, but concerned it will wipe all the data in the process.

Well then ensure you have an up to date backup and then see what happens if you click the "Expand Volume" button.

Note that if the expansion succeeds, you will be running RAID-0 and spanning multiple disks. If either disk fails, you lose everything.

Yes, going to go with the safer option and make separate volumes.

When you select your second disk, you have to click "new volume" on the righthand side. It is located just beneath the x-raid button.

I have my RN102 setup exactly the same way (2 disks setup as 2 seperate volumes).

If one day, I decided to change to RAID 1, can I switch Flex-RAID (JBOD) to X-RAID (RAID-1)?
All data in that second volume are not required anymore or have been backup, I suppose I just need to do the following step:

1. Copy all data out from second volume
2. Remove second volume physically
3. Configure re-enable X-RAID (JBOD) for the first volume
4. Add a new disk (size > volume 1)
5. Select add parity
6. Done - now X-RAID - RAID 1 on both 2TB drive

please correct if i'm wrong

In the OS 6 manual it mention like this, so I suppose for JBOD with only one volume, it okay to switch to X-RAID mode again.

Change from Flex-RAID to X-RAID2 If your system contains only one volume, you can easily switch from Flex-RAID to X-RAID2. Data on the Flex-RAID volume is preserved when you switch to X-RAID2. If your system contains multiple volumes, you must first reconfigure your disks into a single volume. Note: When you switch to X-RAID2 mode, any extra disks installed in your system are automatically reformatted and used for storage expansion. You cannot change the RAID mode of a RAID 0 or RAID 10 volume.

Between step 1 and step 2 you should delete the volume on disk 2 using the GUI first, before removing the disk"
I would also backup the data on disk 1 before you do this.
Although data should be preserved during the switch, I'd recommend having a backup before you do it.