WD RE4 drives - slow writes/impending fail with high SMART values?

Hi I look after a large RAID5 array which is showing very slow writes. I’ve run long smartctltests on all the drives in the array and no errors are mentioned in the summaries. I am suspecting the following 2 drives - can anyone give me some advice please? I am looking at Raw_Read_Error_Rate and Multi_Zone_Error_Rate values and I’m thinking Drive 4 is probably in the worse shape. Here’s smartctl output from the long tests I ran last night:

Drive 0

smartctl 5.42 2011-10-20 r3458 [i686-linux-2.6.18-348.1.1.el5] (local build)
Copyright (C) 2002-11 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital RE4 Serial ATA
Device Model:     WDC WD2003FYYS-02W0B0
Serial Number:    WD-[[DELETED-Privacy]]
LU WWN Device Id: 5 0014ee 057f7a1ee
Firmware Version: 01.01D01
User Capacity:    2,000,398,934,016 bytes [2.00 TB]
Sector Size:      512 bytes logical/physical
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   8
ATA Standard is:  Exact ATA specification draft version not indicated
Local Time is:    Wed Oct 26 08:58:31 2016 BST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
...
SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x002f   200   200   051    Pre-fail  Always       -       0
  3 Spin_Up_Time            0x0027   253   253   021    Pre-fail  Always       -       8791
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       33
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0
  9 Power_On_Hours          0x0032   045   045   000    Old_age   Always       -       40571
 10 Spin_Retry_Count        0x0032   100   253   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       31
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       30
193 Load_Cycle_Count        0x0032   200   200   000    Old_age   Always       -       2
194 Temperature_Celsius     0x0022   103   090   000    Old_age   Always       -       49
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0030   200   200   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   200   001   000    Old_age   Offline      -       0

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%     40567         -
# 2  Extended offline    Completed without error       00%     39384         -
# 3  Short offline       Completed without error       00%     39379         -
# 4  Short offline       Completed without error       00%     39379         -

Drive 4

smartctl 5.42 2011-10-20 r3458 [i686-linux-2.6.18-348.1.1.el5] (local build)
Copyright (C) 2002-11 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital RE4 Serial ATA
Device Model:     WDC WD2003FYYS-02W0B0
Serial Number:    WD-[[DELETED - Privacy]]
LU WWN Device Id: 5 0014ee 600e61468
Firmware Version: 01.01D01
User Capacity:    2,000,398,934,016 bytes [2.00 TB]
Sector Size:      512 bytes logical/physical
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   8
ATA Standard is:  Exact ATA specification draft version not indicated
Local Time is:    Wed Oct 26 08:59:53 2016 BST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
---
SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x002f   200   200   051    Pre-fail  Always       -       46
  3 Spin_Up_Time            0x0027   253   253   021    Pre-fail  Always       -       9250
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       33
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0
  9 Power_On_Hours          0x0032   045   045   000    Old_age   Always       -       40563
 10 Spin_Retry_Count        0x0032   100   253   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       31
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       30
193 Load_Cycle_Count        0x0032   200   200   000    Old_age   Always       -       2
194 Temperature_Celsius     0x0022   099   087   000    Old_age   Always       -       53
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0030   200   200   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   195   195   000    Old_age   Offline      -       1044

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%     40562         -
# 2  Extended offline    Completed without error       00%     40559         -
# 3  Short offline       Completed without error       00%     39368         -

I don’t see any abnormal values on Drive 0, but yeah, Drive 4 looks ill…