We have a Thecus 5200 NAS here at work with five WD Caviar Black 2TB disks in a RADI5 array. Yesterday, disk 4 dropped out of the array, and in the NAS web interface there's a warning about the RAID array being "degraded". When I go into Storage -> Disks, disk 1 and 4 has a warning next to them. When I click on the warnings, this information about the disks are displayed:

Tray Number 4
Model WD2001FASS-00W2B
Power On Hours 2403 Hours
Temperature Celsius 34
Reallocated Sector Count 66
Current Pending Sector 1447
Raw Read Error Rate 61
Seek Error Rate 0
Hardware ECC Recovered N/A

Tray Number 1
Model WD2001FASS-00W2B
Power On Hours 2403 Hours
Temperature Celsius 32
Reallocated Sector Count 0
Current Pending Sector 1465
Raw Read Error Rate 0
Seek Error Rate 0
Hardware ECC Recovered N/A

I'm not really an expert on either disks or RAID arrays. Does this indicate that the fourth disk is damaged, and needs to be replaced? And what about disk number one? It has a warning, but it's still in the array.

Is it safe to add the fourth disk back into the array as a spare? I can't find any way to add it back as a it were before.