How do they keep the helium in?
Two problems spring to mind:
1. HDDs usually have a filtered breather hole to cope with the innards heating up during use. So this one must be built to cope with pressure changes.
2. According to a former student colleague who has worked at BOC these last 25 years, helium is notoriously hard to contain (unlike hydrogen, it's a single-atom molecule and can squeeze through the smallest gap). So even a "hermetically sealed" (see 1 above) drive casing is hard to design as you need special helium-proof gaskets where the case is screwed together, unless they are literally welding it shut.
Get the above wrong and it reverts to an air-filled enclosure (or a partial vacuum when cold, as the heated helium escapes) fairly quickly. Must be a real design headache.