RN214 fails to install disk in Bay3

To resolve the problem discussed in
https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/RN-104-failed-to-start-with-quot-Boot-from-USB-quot-no-FW-

update/td-p/1878838/highlight/false

I ended up buying two RN214s, one (214a) to be populated with the 11Tb 4 disk array I was migrating, and another (214b) to be

populated with a 4Tb zeroed WD red and two other 4Tb WD red I bought at the same time.
The 4TB zeroed disk was used to test 214a, and so it had OS6.10.1 installed at that time.

214b was then started with the zeroed disk in Bay 1 and the other disks in bay 2 & 3. When powered up (welcome message on display), the LEDs for all 3 disks lit up, but the LED for disk 3 went off as soon as the display said 'Booting'; the 214 then started synching the disk in Bay 2. Once finished, Bay 3 never appeared, the Admin-Volume page always showing only Bay 1 & 2.
214b was rebooted a number of times doing set up, and upgrading the OS to 6.10.3, everytime the LED for disk 3 flashing briefly at

start.
Having read the thread
https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/ReadyNAS-214-fails-to-recognise-4th-drive/td-p/1223155

which mentioned the possibility of a dead bay, I powered off the 214 and inserted the disk in Bay 4, seen that this time the LED

remained lit during "Booting", and depowered it immediately after that (but powering down took some time). I then removed the disk

from Bay 4 and placed again Bay 3, but after powering up, NAS is showing the array as degraded (it had already seen a disk in Bay

4, added to the array, but now it has disappeared).

As the disks do not containe any data, followed the advice from the same thread:
https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/ReadyNAS-214-fails-to-recognise-4th-drive/td-p/1223155
and did a factory default using the zeroed disk in Slot 1; after the factory default, OS was 6.10.3 (as expected)
I also did a Quick zeroing on the 2 x4TB WD Red using WD Lifeguard to get to an "as new" identical stage.
After some trivial networking issues ("no IP address"), I could enter admin page and do some basic configuration. Then:

Powered down, moved HD1 to Bay 3 to test it & reboot - All OK, disk is seen.
Configured Network to 192.168.2.4 & reboot - All OK
Powered down, moved the HD1 back to Bay 1 & reboot - All OK
Powered Down, added HD2 in Bay 2 (as before) and reboot. Display shows "Recover data 0.00% >> (16 April 2020 14:00)..............Bay2 Synched, Full DATA Raid 1 (17 April 2020 03:00) - All OK
Powered Down, added HD3 in bay 3 (as before) and reboot. LED 123 on until the Display said "Booting", then LED3 goes off. Disk 3 NOT shown on the Admin-Volume page - Nope, same situation as before.

To Summarize:
HD3 is functional (recognized when inserted in Bay 4)
Bay 3 is functional (HD1 booted from Bay3)
What next ?

Many thanks in advance, Berillio