First thing to do is swap the data cable from the drive in question to a known good cable. If nothing else just switch it with the data cable from a different drive for now. Also try plugging in a different power plug from the PSU.
Open up a command prompt (or elevated command prompt in Vista or Win 7).
Type chkdsk /r x:
and hit enter. X = the drive letter of the drive to test. You will be prompted that it can't run now and do you want to run it the next time you reboot respond with a Y
then reboot the computer. Depending on how large the drive is and how many problems it finds this could take up to an hour or more to complete. This is MUCH more exhaustive a test than what Windows will normally prompt you for when it finds a problem.
Once that is done, download Victoria
and use it to check the SMART status of the drive and to run the TEST. You don't have to install it just click on the executable once you have extracted the archive. It's designed for Win 2k and XP but it will also run in Vista and 7 although you might get a few error messages when you start it up. Just keep clicking on OK until you get to the main screen. Make a screenshot of the smart status and of the completed TEST results screen to compare later. Ideally you won't see any entries higher than 50. 200 is acceptable, but not ideal. Anything higher is bad, but the next step could possibly fix it. Also make a note of the temp of the hard drive during the tests. It should be somewhere in the 30C's. Anything over 40C means you need to increase the cooling for the drive. Seagates are known to run hot, especially when they are having issues.
Now backup all of the data and use SeaTools to zero fill the entire drive, then do a COMPLETE format (not a quick format). DO NOT use an imaging program like Acronis True Image, Ghost, etc. for this step. You want to copy the files manually so that when you restore the data you don't want the image restoration process to overwrite the format you will be doing later in the process. Now run Victoria again and compare the SMART and TEST results to see if there is any change. If the results are worse then the drive is definitely failing. You might want to run several zero fills to see if that will push it over the edge.
If the results are better restore the data then wait a few days and run Victoria again and compare the results.Edited by PapaSmurf - 11/1/10 at 2:27pm