> Running a long SMART self-test will do a good job of examining the drive 
> hardware and ought to give you a better picture of what is going on.  It 
> would not surprise me if this slow drive has a bunch of bad sectors which are 
> being transparently corrected using ECC and the automatic hotspare mechanisms 
> that modern drives do.  Even though the drive hasn't reported a failure, the 
> correction process would cause a very noticable slowdown, similar to what 
> you've described is actually happening.

I already use smartmontools with automatic full test once a week. Both of the 
drives check out just fine. Neither of the drives have a single bad (or 
reallocated) sector. They have about 2000 hours on them. While I can't 
completely rule out hardware failure, I'd find it odd to have two HDDs with 
the exact same fault.

-- 
Ari Sovijärvi
http://apz.fi/