Seems like the firmware doesn’t recognized the replaced hard disk and the web interface still complains that the RAID 5 is degraded. Telnet into the DNS-343 and check the status of /dev/md0 (mdadm –detail /dev/md0). The RAID 5 is running on 3 disks and the 4th disk has not been added back into the RAID array.
Seems like /dev/sdd2 has not been added back to the array. Hot added the /dev/sdd2 (mdadm –add /dev/md0 /dev/sdd2) and viola! The RAID 5 array is complete again. The DNS-343 is happily syncing the data when checked (mdadm –detail /dev/md0).