Here's a proven example of what a bad hard drive can do, it was technically functioning OK in a RAID array but the system became extremely low and the load become high and IOWAIT was even higher and I always thought it was a bad application.  The truth is that this failing 1TB Hitachi has slowly gotten worse and caused huge slowdowns, (eg. 100% load on Thunderbird waiting for e-mails to load etc..).  After swapping it out, tabs change instantly, emails are not lagged, and Firefox does slow down or lag.  Basically generally slowness in your system that can't be explained or corrected is often due to a bad hard drive causing high IOWAIT which results in extreme lag.  

Look below at all of the UDMA_CRC_ERRORS

199 UDMA_CRC_Error_Count    0x000a   200   200   000    Old_age   Always       -       122

And the other error messages from the SMART log:

Error 122 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 00 00 00 00 00

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  ec 00 00 00 00 00 a0 08  25d+05:58:47.630  IDENTIFY DEVICE
  60 18 00 27 54 fe a0 04  25d+05:58:46.675  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 04  25d+05:58:45.623  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 ff  25d+05:58:43.621  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 ff  25d+05:58:43.611  READ FPDMA QUEUED

 sudo smartctl -a /dev/sdc
smartctl 5.40 2010-03-16 r3077 [i686-pc-linux-gnu] (local build)
Copyright (C) 2002-10 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF INFORMATION SECTION ===
Device Model:     Hitachi HDS721010CLA332
Serial Number:    JP2940HQ3ZY7KH
Firmware Version: JP4OA3EA
User Capacity:    1,000,204,886,016 bytes
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   8
ATA Standard is:  ATA-8-ACS revision 4
Local Time is:    Mon Apr  8 17:59:38 2013 PDT
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x84)    Offline data collection activity
                    was suspended by an interrupting command from host.
                    Auto Offline Data Collection: Enabled.
Self-test execution status:      (   0)    The previous self-test routine completed
                    without error or no self-test has ever
                    been run.
Total time to complete Offline
data collection:          (10399) seconds.
Offline data collection
capabilities:              (0x5b) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    Offline surface scan supported.
                    Self-test supported.
                    No Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003)    Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01)    Error logging supported.
                    General Purpose Logging supported.
Short self-test routine
recommended polling time:      (   1) minutes.
Extended self-test routine
recommended polling time:      ( 173) minutes.
SCT capabilities:            (0x003d)    SCT Status supported.
                    SCT Error Recovery Control supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

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     0x000b   099   099   016    Pre-fail  Always       -       2
  2 Throughput_Performance  0x0005   135   135   054    Pre-fail  Offline      -       97
  3 Spin_Up_Time            0x0007   135   135   024    Pre-fail  Always       -       238 (Average 321)
  4 Start_Stop_Count        0x0012   100   100   000    Old_age   Always       -       31
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x000b   100   100   067    Pre-fail  Always       -       0
  8 Seek_Time_Performance   0x0005   138   138   020    Pre-fail  Offline      -       31
  9 Power_On_Hours          0x0012   098   098   000    Old_age   Always       -       20049
 10 Spin_Retry_Count        0x0013   100   100   060    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       31
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       43
193 Load_Cycle_Count        0x0012   100   100   000    Old_age   Always       -       43
194 Temperature_Celsius     0x0002   142   142   000    Old_age   Always       -       42 (Lifetime Min/Max 21/60)
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x000a   200   200   000    Old_age   Always       -       122

SMART Error Log Version: 1
ATA Error Count: 122 (device log contains only the most recent five errors)
    CR = Command Register [HEX]
    FR = Features Register [HEX]
    SC = Sector Count Register [HEX]
    SN = Sector Number Register [HEX]
    CL = Cylinder Low Register [HEX]
    CH = Cylinder High Register [HEX]
    DH = Device/Head Register [HEX]
    DC = Device Command Register [HEX]
    ER = Error register [HEX]
    ST = Status register [HEX]
Powered_Up_Time is measured from power on, and printed as
DDd+hh:mm:SS.sss where DD=days, hh=hours, mm=minutes,
SS=sec, and sss=millisec. It "wraps" after 49.710 days.

Error 122 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 00 00 00 00 00

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  ec 00 00 00 00 00 a0 08  25d+05:58:47.630  IDENTIFY DEVICE
  60 18 00 27 54 fe a0 04  25d+05:58:46.675  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 04  25d+05:58:45.623  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 ff  25d+05:58:43.621  READ FPDMA QUEUED
  60 18 00 27 54 fe a0 ff  25d+05:58:43.611  READ FPDMA QUEUED

Error 121 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 09 36 54 fe 05

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 18 00 27 54 fe 40 08  25d+05:58:43.528  READ FPDMA QUEUED
  60 10 00 bf 6c fe 40 08  25d+05:58:43.479  READ FPDMA QUEUED
  60 20 00 87 3e fd 40 08  25d+05:58:43.427  READ FPDMA QUEUED
  60 18 00 ff 1b fe 40 08  25d+05:58:43.372  READ FPDMA QUEUED
  60 20 00 67 fd fd 40 08  25d+05:58:43.345  READ FPDMA QUEUED

Error 120 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 01 de e7 fc 05

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 10 00 cf e7 fc 40 08  25d+05:58:13.286  READ FPDMA QUEUED
  60 10 00 af 83 fc 40 08  25d+05:58:13.267  READ FPDMA QUEUED
  ef 10 02 00 00 00 a0 08  25d+05:58:13.266  SET FEATURES [Reserved for Serial ATA]
  27 00 00 00 00 00 e0 08  25d+05:58:13.265  READ NATIVE MAX ADDRESS EXT
  ec 00 00 00 00 00 a0 08  25d+05:58:13.261  IDENTIFY DEVICE

Error 119 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 01 be 83 fc 05

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  60 10 00 af 83 fc 40 08  25d+05:58:09.177  READ FPDMA QUEUED
  ef 10 02 00 00 00 a0 08  25d+05:58:09.176  SET FEATURES [Reserved for Serial ATA]
  27 00 00 00 00 00 e0 08  25d+05:58:09.175  READ NATIVE MAX ADDRESS EXT
  ec 00 00 00 00 00 a0 08  25d+05:58:09.171  IDENTIFY DEVICE
  ef 03 42 00 00 00 a0 08  25d+05:58:09.170  SET FEATURES [Set transfer mode]

Error 118 occurred at disk power-on lifetime: 20037 hours (834 days + 21 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 00 00 00 00 00

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  ec 00 00 00 00 00 a0 08  25d+05:57:34.494  IDENTIFY DEVICE
  60 10 00 af 83 fc a0 04  25d+05:57:33.538  READ FPDMA QUEUED
  60 10 00 af 83 fc a0 04  25d+05:57:32.486  READ FPDMA QUEUED
  60 10 00 af 83 fc a0 ff  25d+05:57:30.486  READ FPDMA QUEUED
  60 10 00 af 83 fc a0 ff  25d+05:57:30.477  READ FPDMA QUEUED

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%       207         -

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.
 

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