Thanks Phil,
I should note that the drives are labelled "enterprise", purchased from a hw RAID vendor (ACNC.com).

On 17-12-2013 12.55 -0500, Phil Turmel wrote:

Please post the output of "smartctl -x" for both of these drives.

The Centos5 smartctl (from smartmontools rpm) doesn't support the -x option. However, it's apparently equivelent to:
smartctl -H -i -g all -c -A -f brief -l xerror,error -l xselftest,selftest -l selective -l directory -l scttemp -l scterc -l devstat -l sataphy 

Centos5 smartctl supports the following:
 smartctl -H -i -c -A -l error -l selftest -l selective -l directory -l scttemp -l scttempsts -l scttemphist

... and I enclosed the output for sda and sdb.
If you think it would be useful to have the additional options (provided by -x), then let me know, and I'll try to build it.

timeout mismatches combined with lack of scrubbing.

I've read about mismatches, but not about scrubbing. I'll investigate this.
What program/options do your weekly scrub?
 

Maybe not.  Please tell us you know all about error recovery timeouts

Instead of stopping the sync, I decided to slow it down:
echo 1001 > /proc/sys/dev/raid/speed_limit_max

and the timeout mismatch problem commonly encountered with
consumer-grade hard drives.  Otherwise, you might want search the list
archives for various combinations of the keywords "scterc", "error
recovery", "timeout mismatch", "URE", and/or "bit error rate".

I'm not a big fan of Seagate (enterprise or not). The drives I purchased before these (~2008) needed to have firmware updates to prevent bricking. Sigh.

Thanks for your help and search tips.
best,
Julie
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