A rubber composition for HDD top cover gaskets is disclosed. The rubber composition comprises a specific ethylene/α-olefin/non-conjugated polyene copolymer [A], a specific organopolysiloxane [B] and an SiH group-containing compound [C], with the weight ratio of [A] : [B] being 100:0 to 5:95. A HDD top cover gasket according to the present invention comprises the composition. A hard disk drive of the present invention includes the gasket. The present invention also provides a crosslinkable rubber composition that has good high-speed moldability and can give HDD top cover gaskets having low hardness and being excellent in heat resistance, compression set resistance and long-term sealing properties. Further provided are a gasket from the composition and a hard disk drive including the gasket.