There are many openings in a hard drive which are currently sealed with epoxy or plastic/foil adhesives. The biggest is the seal between the top cover and the drive body but there are also many holes used during manufacturing. If you examine a hard drive's casing you can see where these have been sealed with circular stickers. Less visible are the openings for the spindle motor and servo/channel control lines.
Making all of these openings impermeable to He is not a minor change and will likely significantly increase the cost of the drive.