0.05 gram Helium assuming: (1) half of drive volume is used for platters, (2) at 15° Celcius (3) at 1 atmosphere (I'm guessing not pressurised since that would

0.05 gram Helium assuming: (1) half of drive volume is used for platters, (2) at 15° Celcius (3) at 1 atmosphere (I'm guessing not pressurised since that would defeat purpose of using Helium, although might be less than 1 atmosphere).. 0.376 litres for total 3.5-inch HDD volume from my first Google result: "Width: 101.6 mm, Height: 25.4 mm, Length: 146 mm". Actual volume used for platters is less than that. Helium is light ~ 0.169 g/litre from "0.169 kg of Helium is 0.999 m3 at 15°C": https://microsites.airproducts.com/gasfacts/helium.html https://microsites.airproducts.com/gasfacts/helium.html Note that I think comment assumed one litre of normal air not Helium: "The density of dry air is 1.2929 g/litre at STP".