[blue68f100]

The top drive is the main drive in JBOD.

I always recomend the unit to start in RAID 1. This way the OS is on both drives. So now you can remove a drive.
I recommend powering down and remove the top drive and install the new drive.

When the units boots it will transfer the OS but will not mirror.
If you have data on the mirror drive. Now is the time the data transfer.
Power down and replace the lower drive.
Reboot the server.

Now you need to change the drive config back to JBOD. for 1 cycle.
Check to see if drive cap is right if not issue the secret cmd "co de format xxxxx /reinit" to correct.

If all good you can set the raid1 backup.