A word of warning on SMART diagnostics

by: fearless

Last modified: Apr 26, 2016 9:43 PM

0 773 Last modified Apr 26, 2016 9:43 PM

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.