I use a waterproof Ortlieb Office Bag pannier with the laptop in a padded sleeve in the bag. It works great. Hard drives do park the head so it is less susceptible to damage (but not impervious). My personal laptop has an SSD drive, but my work one doesn't (I didn't get to pick it). I recommend backing up the data you care about regularly as hard drives fail for a lot of reasons even if you are careful with them.