hi Phil,
thanks again for your help. It was surprisingly easy to install the latest smarmontools.

On 17-12-2013 14.43 -0500, Phil Turmel wrote:

I was interested in the reallocation counts, the current pending
sectors, and the scterc timeouts.  The latter were not present, and are
important.

ID# ATTRIBUTE_NAME          FLAGS    VALUE WORST THRESH FAIL RAW_VALUE
  5 Reallocated_Sector_Ct   PO--CK   100   100   036    -    3
197 Current_Pending_Sector  -O--C-   100   100   000    -    1
SCT Error Recovery Control:
           Read:    100 (10.0 seconds)
          Write:    100 (10.0 seconds)

(I also attached the full output)

I verified that a weekly scrub is performed via cron (default with Centos5), and there were no errors detected prior to the sync. The output is included in syslog reports. 

But /dev/sdb has three relocations and only one pending error.  That's
an old drive, but not sick.  I'd be concerned that there're other
hardware issues in your system if the timeout issue is not part of the
problem.

Should I run the sync (mdadm -a) in verbose mode? If so, what is the best way to terminate the current sync? By failing/removing /dev/sda?

Thanks in advance!
best,
Julie
----