[CentOS] Strangely slow disk

I have a 1.5TB Western Digital hard disk (WD15EADS-00R6B0)
on my CentOS-5.7 server, which has become incredibly slow
for some operations, eg rsync, BackupPC archive, e2fsck,
although it seems to work fine for ordinary file operations,
and "smartctl -a /dev/sdb" does not report any errors.

For example, running "e2fsck -p /dev/sdb5" on a 250GB partition
on this disk took over 24 hours to complete,
as it did on another partition.

I wonder if anyone can offer an explanation,
or suggest a remedy?


-- 
Timothy Murphy  
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College Dublin