It would be interesting to check your drive's SMART data to see if that has reported any errors. You might be able to do this with smartctl from the smartmontools package, depending on which USB bridge you have. I recently uploaded a new version of the package to unstable, but I do not know if/when that might appear in Raspbian (which I presume you are using)
-- Jonathan Dowland Please do not CC me, I am subscribed to the list.
