LS520D RAID rebuilding randomly

[sicarumba]

Hi all

I have an LS520D NAS and it seems to like rebuilding the RAID for no apparent reason. It probably happens every 5 or 6 weeks, no other alerts or obvious symptoms. I only noticed because I caught sight of the flashing red LED.

Originally it had a pair of 2TB drives, after the second one failed I took the opportunity to upgrade them both to 4TB. I backed all my data up, destroyed the 2TB RAID, replaced the drives, formatted them and created a new RAID. I then restored all my data manually. I don't know that this rebuilding thing definitely wasn't happening before I upgraded the drives since I previously had the NAS positioned such that I couldn't see the LED, I moved it when I upgraded. I can't see anything in the SMART info which would suggest one drive is vastly different to the other, though I admit I don't fully understand most of the information I am seeing. These are my current values :

Drive 1
ID   Attribute   Status   Current   Worst   Threshold   Raw
1   Raw_Read_Error_Rate   OK   81   64   6   133076420
3   Spin_Up_Time      OK   96   95   0   0
4   Start_Stop_Count   OK   76   76   20   25388
5   Reallocated_Sector_Ct   OK   100   100   10   0
7   Seek_Error_Rate      OK   77   60   45   54440162
9   Power_On_Hours      OK   90   90   0   9311
10   Spin_Retry_Count   OK   100   100   97   0
12   Power_Cycle_Count   OK   100   100   20   17
183   Runtime_Bad_Block   OK   100   100   0   0
184   End-to-End_Error   OK   100   100   99   0
187   Reported_Uncorrect   OK   100   100   0   0
188   Command_Timeout      OK   100   100   0   0
189   High_Fly_Writes      OK   100   100   0   0
190   Airflow_Temperature_Cel   OK   61   57   40   39
191   G-Sense_Error_Rate   OK   100   100   0   0
192   Power-Off_Retract_Count   OK   100   100   0   8
193   Load_Cycle_Count   OK   88   88   0   25778
194   Temperature_Celsius   OK   39   43   0   39
195   Hardware_ECC_Recovered   OK   81   64   0   133076420
197   Current_Pending_Sector   OK   100   100   0   0
198   Offline_Uncorrectable   OK   100   100   0   0
199   UDMA_CRC_Error_Count   OK   200   200   0   0
240   Head_Flying_Hours   OK   100   253   0   261267155590476
241   Total_LBAs_Written   OK   100   253   0   26870756368
242   Total_LBAs_Read      OK   100   253   0   1990032574899

Drive 2
ID   Attribute   Status   Current   Worst   Threshold   Raw
1   Raw_Read_Error_Rate   OK   76   64   6   35567868
3   Spin_Up_Time      OK   95   95   0   0
4   Start_Stop_Count   OK   76   76   20   25376
5   Reallocated_Sector_Ct   OK   100   100   10   0
7   Seek_Error_Rate      OK   78   60   45   62689475
9   Power_On_Hours      OK   90   90   0   9302
10   Spin_Retry_Count   OK   100   100   97   0
12   Power_Cycle_Count   OK   100   100   20   16
183   Runtime_Bad_Block   OK   100   100   0   0
184   End-to-End_Error   OK   100   100   99   0
187   Reported_Uncorrect   OK   100   100   0   0
188   Command_Timeout      OK   100   100   0   0
189   High_Fly_Writes      OK   100   100   0   0
190   Airflow_Temperature_Cel   OK   57   51   40   43
191   G-Sense_Error_Rate   OK   100   100   0   0
192   Power-Off_Retract_Count   OK   100   100   0   8
193   Load_Cycle_Count   OK   88   88   0   25554
194   Temperature_Celsius   OK   43   49   0   43
195   Hardware_ECC_Recovered   OK   76   64   0   35567868
197   Current_Pending_Sector   OK   100   100   0   0
198   Offline_Uncorrectable   OK   100   100   0   0
199   UDMA_CRC_Error_Count   OK   200   200   0   0
240   Head_Flying_Hours   OK   100   253   0   82648055684432
241   Total_LBAs_Written   OK   100   253   0   15155726943
242   Total_LBAs_Read      OK   100   253   0   13311692594788

Does anyone see any issues here, or have any other suggestions I could check please?