I have a drive that was disappearing out of my raid array, so I put in an RMA and replaced the drive. All was a well for a couple weeks and now I am seeing checksum errors after running a scrub that are related to the same drive that I just replaced. I also did not see any SMART errors displayed. 
My weekly scrub has run a couple times since I replaced the drive and I did a zpool clear and the checksum errors returned.
I guess its possible I could have a bad refurbished drive that I received from seagate.
I have attached info below.
ZPOOL STATUS
Camcontrol devlist
glabel status
GPART SHOW
	
		
			
		
		
	
			
			My weekly scrub has run a couple times since I replaced the drive and I did a zpool clear and the checksum errors returned.
I guess its possible I could have a bad refurbished drive that I received from seagate.
I have attached info below.
ZPOOL STATUS
Code:
[root@freenas] ~# zpool status -v
  pool: Data
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-9P
  scan: scrub repaired 419M in 7h48m with 0 errors on Wed May  8 10:48:35 2013
config:
        NAME                                            STATE     READ WRITE CKS                                                     UM
        Data                                            ONLINE       0     0                                                          0
          raidz2-0                                      ONLINE       0     0                                                          0
            gptid/e902c40d-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e91172b0-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e91f0e06-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0 19.                                                     2K
            gptid/f96ec369-a47c-11e2-863b-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e93a03e7-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e94867a0-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e956b27d-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
            gptid/e964d6cf-3326-11e1-acd0-e06995ebe0de  ONLINE       0     0                                                          0
errors: No known data errors
Camcontrol devlist
Code:
[root@freenas] ~# camcontrol devlist <ATA ST2000DL003-9VT1 CC32> at scbus0 target 0 lun 0 (pass0,da0) <ATA ST2000DL003-9VT1 CC32> at scbus0 target 1 lun 0 (pass1,da1) <ATA ST2000DL003-9VT1 CC3C> at scbus0 target 2 lun 0 (pass2,da2) <ATA ST2000DL003-9VT1 CC3C> at scbus0 target 3 lun 0 (pass3,da3) <ATA ST2000DL003-9VT1 CC32> at scbus0 target 5 lun 0 (pass4,da4) <ATA ST2000DL003-9VT1 CC32> at scbus0 target 6 lun 0 (pass5,da5) <ATA ST2000DL003-9VT1 CC32> at scbus0 target 7 lun 0 (pass6,da6) <ATA ST32000542AS CC34> at scbus0 target 8 lun 0 (pass7,da7) <ATAPI DVD A DH16A3L 8H1F> at scbus3 target 0 lun 0 (pass8,cd0) <ST31500341AS CC1H> at scbus4 target 0 lun 0 (pass9,ada0) <ADATA USB Flash Drive 1.00> at scbus7 target 0 lun 0 (pass10,da8)
glabel status
Code:
[root@freenas] ~# glabel status
                                      Name  Status  Components
gptid/e964d6cf-3326-11e1-acd0-e06995ebe0de     N/A  da0p2
gptid/e956b27d-3326-11e1-acd0-e06995ebe0de     N/A  da1p2
gptid/e94867a0-3326-11e1-acd0-e06995ebe0de     N/A  da2p2
gptid/e93a03e7-3326-11e1-acd0-e06995ebe0de     N/A  da3p2
gptid/e91f0e06-3326-11e1-acd0-e06995ebe0de     N/A  da4p2
gptid/e91172b0-3326-11e1-acd0-e06995ebe0de     N/A  da5p2
gptid/e902c40d-3326-11e1-acd0-e06995ebe0de     N/A  da6p2
gptid/f96ec369-a47c-11e2-863b-e06995ebe0de     N/A  da7p2
                             ufs/FreeNASs3     N/A  da8s3
                             ufs/FreeNASs4     N/A  da8s4
                            ufs/FreeNASs1a     N/A  da8s1a
                    ufsid/5144fed8f696ca23     N/A  da8s2a
                            ufs/FreeNASs2a     N/A  da8s2a
gptid/42c1112e-b5f8-11e2-9553-e06995ebe0de     N/A  ada0p2
GPART SHOW
Code:
[root@freenas] ~# gpart show
=>        34  3907029101  da0  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da1  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da2  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da3  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da4  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da5  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da6  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>        34  3907029101  da7  GPT  (1.8T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  3902834703    2  freebsd-zfs  (1.8T)
=>      63  30883777  da8  MBR  (14G)
        63   1930257    1  freebsd  [active]  (942M)
   1930320        63       - free -  (31k)
   1930383   1930257    2  freebsd  (942M)
   3860640      3024    3  freebsd  (1.5M)
   3863664     41328    4  freebsd  (20M)
   3904992  26978848       - free -  (12G)
=>      0  1930257  da8s1  BSD  (942M)
        0       16         - free -  (8.0k)
       16  1930241      1  !0  (942M)
=>      0  1930257  da8s2  BSD  (942M)
        0       16         - free -  (8.0k)
       16  1930241      1  !0  (942M)
=>        34  2930277101  ada0  GPT  (1.4T)
          34          94        - free -  (47k)
         128     4194304     1  freebsd-swap  (2.0G)
     4194432  2926082703     2  freebsd-zfs  (1.4T)