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A word of warning on SMART diagnostics

I've just had a near-death experience with a RAID0 set of 4x3TB WD RE3s inside my speed-bumped 5,1 (booting off a separate SSD). They're just over 3 years old (Jan 2013), and have performed flawlessly til this morning, when the RAID mounted read-only - my only clue that there was an issue . Fortunately due to a robust backup programme, I've lost no data of any importance (the speed for big media is worth the risk, but requires a robust approach) and it could have been very different.

 

On a reboot the RAID failed to mount with a "can't repair" error in Disk Utility, and DiskWarrior felt the same way. However eventually it mounted read-only again and I was able to immediately offload the small amount of data since the last incremental backup, then set about attempting to clone it as a further protection before doing a major restore of ~10TB from another RAID.

 

A closer look at the info in Disk Utility reveals that one drive is indeed on the way out - enough to bring down an Apple RAID set but not enough to trigger the SMART Diagnostics into reporting a fault. FYI, here it is, from the log:

2016-04-27 14:59:12 +1200:

  Name : WDC WD3000FYYZ-01UL1B0 Media

  Type : Disk

  Partition Map Scheme : GUID Partition Table

  Disk Identifier : disk3

  Media Name : WDC WD3000FYYZ-01UL1B0 Media

  Media Type : Generic

  Connection Bus : SATA

  Connection ID : "Bay 4"

  Device Tree : IODeviceTree:/PCI0@0/SATA@1F,2/PRT3@3/PMP@0

  Bay : "Bay 4"

  Writable : Yes

  Ejectable : No

  Location : Internal

  Total Capacity : 3 TB (3,000,592,982,016 Bytes)

  Disk Number : 3

  Partition Number : 0

  S.M.A.R.T. Status : Verified

  Raw Error Rate : 00000000006D

  Spinup Time : 000000002D50

  Start/Stop Count : 000000000934

  Reallocated Sectors : 0000000002B1

  Seek Error Rate : 000000000000

  Power-On Hours : 0000000063C2

  Spinup Retries : 000000000000

  Calibration Retries : 000000000000

  HDD Temperature : 00000000003A

  Reallocated Sector Events : 0000000001E5

  Current Pending Sectors : 0000000000DD

  Offline Scan Uncorrectable Sectors : 0000000000CA

  CRC Error Rate : 000000000000

  Multi-Zone Error Rate : 000000000455

 

So clearly errors there - the other drives have 0 relocated sectors, and 0 raw errors to report. I love this level of granularity, which you don't get with the cheaper Green drives. Good to know you get something for your money - but not better automatic diagnostics. And a 5 year warranty of course... so I get a replacement.

 

The moral of the story: just because nothing's flashing red in Disk Utility doesn't mean your drives are fine - one of mine was not, and it's brought the house down for a day.

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