Hello, my name is***** hope I can help!
You may be mistaken, or the software is rendering an incorrect configuration with regards ***** ***** RAID 5 on 2 drives, and another RAID 5 on the other two drives. RAID 1 requires at least 2 drives, and RAID 5 requires at least 3 drives in the RAID group.
Essentially, you can't have 2 drives in a RAID5. Maybe these drives are in a RAID1 configuration?
Regardless, if you can still access the data on the volumes I'd advise you to back it up before we attempt any steps. Anytime you shutdown NASs or disk drawers you run the risk of data loss. Once the data is backed up, power the 343 down and remove the faulty disk. Insert the fresh disk and power the unit back on. The new drive should be recognized and the data should begin copying over from the (still-good) drive from the same volume group.
From this point you may continue to use the 343 as normal. However, in some cases (based off of some reviews I've read) the firmware on the drive, or the hardware may be bad. This could result in the new drive not being recognized or the copy-back not happening. Unfortunately, there is no way to repair if this occurs. You can attempt to restart the device to try and kick-start the copy, but even that may not work.
In the future, I'd recommend using a RAID5 configuration. You will lose an entire disk's worth of storage due to parity, but you gain some redundancy.
Four 1TB drives in a RAID5 configuration should net you ~3TB of usable space, and allow for a single drive failure with no data loss.
With that consideration in mind, it may be beneficial to you to reconfigure the 343 when you remove and replace the drive; that is up to you. Please let me know if you require additional assistance.