Jeff Garzik wrote:

In some cases, NCQ firmware may be broken.  There is a Maxtor firmware 
id, and some Hitachi ids that people are leaning towards recommending be 
added to the libata 'horkage' list.

Western Digital "Raptor" drives (the 10K rpm things) are also somewhat
borked in NCQ mode, depending on the application.

Their firmware turns off all drive readahead during NCQ.
This makes them very good for an email/news server application,
but also causes them to suck for regular desktop applications.

Because of this, they use special software drivers under MSwin
which detect large sequential accesses, and avoid NCQ during such times.

Cheers

-ml