Adding HDD to existing External RAID 1

Hello -

Need some advice please.

I have an external RAID 1 (2 x 18tb HDD) and am adding an additional 18tb HDD to this configuration.

I am using Disk Utility to do this. I added the HDD as a "Member". The status box pops up and it states "Adding disk6 to RAID set "HGST18 R1AB" (the name of my current RAID 1 set). Below that, there is a colored line going back and forth, left to right; and below that it states "Running...". Then below that, is a down arrow for Hide or Show Details … but nothing shows in the status area when I click on the down arrow to either "Show Details" or "Hide Details".

The lights on my external HDD box only shows the power light. I usually see the activity light(s) of the HDD's when I access them. Since I started this … I do hear an occasional "click" from a HDD, but no activity light.

It's been going on for about 5 hours now and I’m not sure if it’s really doing anything.

So, is this normal?

I realize 18tb takes a while to format … so maybe I just wait it out and see what happens?

Or maybe I did something wrong in my process to do this?

Any advice would be greatly appreciated.

My Setup:

Computer:

MacBook Pro

M1 Max

64gb RAM

Sonoma 14.1

External HDD Box:

OWC ThunderBay 4 Four-Bay External Drive Enclosure with Dual Thunderbolt 3 Ports

Cable connecting from MBP to Box is a Thunderbolt 4 cable

HDD’s: All the same brand (HGST) and size (18tb)

2x18tb in RAID 1

1x18tb formatted as Mac OS Extended (Journaled) ... (this is the drive I am trying to add to the existing RAID 1 set.)

Thanks!!!