Thecus N5200, disk has dropped out of RAID5

Question

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.

Answer 1

You should replace tray 4 disk - it can go down anytime (probably it is aleready down when it dropped out of array). When raw read error rate or reallocated sector count rise it's time to replace hdd before total failure. Also prepare another spare disk for tray 1 - current pending sector means that some unstable sectors waits for remapping, when this number is not decreasing over time, try to format it (outside of NAS), sometimes it helps. After replacing hdd prepare for long array rebulding - it can take even few days.

Answer 2

I would recommend reading this wikipedia article about a featerure called TLER http://en.wikipedia.org/wiki/Time-Limited_Error_Recovery as we had problems with WD drives dropping out our our Thecus RAID arrays.

WD have changed the default settings for their Black drives and so it is not always obvious if TLER is enabled or disabled. Suffice it to say, if it is set wrong you will experience a higher number of drive drop outs and with RAID 5 only having one spare disk you would run the risk of losing your data