I get this problem sometimes with my 40gb Seagate which used to be the Primary HDD when I first bought the computer, now I have a 120gb Western Digital as my Primary and I use the Seagate as a slave. Sometimes, usually when I am trying to extract an image over 3gig or copy something rather large to the Seagate I get the usual Cyclic Redundancy Check, I then restart the computer and on startup where it says 'Checking Hardware' or whatever it says, the Seagate isn't listed, so I have to turn the computer off, open it up and just pull out the IDE cable and stick it back in again and it works fine for a couple of months until I have to do it again.
Maybe I should just buy a new computer, but then again, need a car, job, money, etc. bah, stuff it.