It is indeed a long thread -- the first portion of the thread was closed because the mods said it was causing some browsers to time out. Here is the first portion:
http://discussions.apple.com/thread.jspa?threadID=2054387&start=0&tstart=0
Some posters have reported that they tracked the cause of the problem to the SATA cable. When they replaced the cable or wrapped it with aluminum foil and electrical tape, the problem went away. So shielding or routing of the cable may be causing the problem. Also, people have posted that drives that might exhibit long interruptions in data transfer when installed in June 2009 revision MBPs running EFI 1.7 firmware (that enabled SATA 2 speed) worked perfectly in an external USB enclosure. Consequently, it's possible that many perfectly good drives were returned to the retailer.
I don't believe that Apple has ever explained why the June 2009 13" and 15" MBPs may exhibit this problem, so it appears unlikely that it will ever be addressed -- except possibly when Apple releases the next revision of the MBP. While Steve Jobs may express disdain for netbooks, at least netbooks can have their hard drives upgraded without worrying about unreliable data transfer over the SATA 2 bus.