It turns out the problem is caused by hdparm's RAID_WORKAROUND option in
/etc/default/hdparm. I'm still not sure whether it's a kernel bug or a
bug in hdparm, but at least disabling this option fixes the problem....

--
Ing. A.C.J. van Amersfoort (Arno)
Electronics & ICT Engineer
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Leiden Institute of Physics (LION), Electronics Department (ELD)
Huygens Laboratory (Room 1007), Leiden University
Postal Address: P.O. Box 9504, 2300 RA Leiden
Visit Address : Niels Bohrweg 2, 2333 CA Leiden
The Netherlands
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Phone : +31-(0)71-527.1894
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E-mail : a.c.j.van.amersfoort@eld.physics.leidenuniv.nl
Homepage : http://rocky.eld.leidenuniv.nl