Since my last comment, I have done some more testing. While performance
is generally much better than it was a year ago, it still isn't quite as
fast as it should be on two of my drives, and these drives don't appear
to be running in any UDMA mode. The other one seems to perform a bit
better, and has udma5 enabled. Here's the relevant output of hdparm -i
and hdparm -tT on each of my three drives (the full output is attached):

# Maxtor 80GB SATA
 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 
 Timing buffered disk reads:  170 MB in  3.01 seconds =  56.39 MB/sec

# Seagate 80GB SATA
 UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5 
 Timing buffered disk reads:  224 MB in  3.01 seconds =  74.42 MB/sec

# Western Digital 500GB SATA
 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 
 Timing buffered disk reads:  206 MB in  3.02 seconds =  68.15 MB/sec

Copying a 700MB ISO image between each of these drives takes between 11
and 20 seconds, with the mean average being about 15 seconds; the faster
the transfer, the higher the CPU usage (up to 30% in some cases, much
higher than I would expect considering DMA is supposed to be in use). I
have attached details of this test as well (9 copy operations in all),
in case this is of any use.

Of course, I don't really know what speeds I'm supposed to get on these
drives - I have never paid much attention to copy speed until this
problem occured. It's possible that these speeds are normal (they're
certainly quick enough that this isn't as big an issue as it was),
though the fact that UDMA still seems to be disabled on two drives makes
me think something is still not working as it should.

** Attachment added: "hdparm -i and hdparm -tT output for each of my SATA 
drives"
   
http://launchpadlibrarian.net/12917915/C%3A%5CDocuments%20and%20Settings%5Ckhvt356.EMEA%5CDesktop%5Clspci%5Chdparm.txt

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Slow SATA performance
https://bugs.launchpad.net/bugs/119730
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