A rack server includes a chassis having a front and a rear ends which are divided by a partition board into a first and a second chambers; a mother board module, located in the first chamber; a hard drive module, disposed in the second chamber. The hard drive module includes a hard drive cage, slidably disposed in the second chamber from the front end, in which the hard drive cage has a first and a second sides parallel to the partition board and the second side has a hard drive port; a hard drive backplane, vertically fixed to the first side of the hard drive cage; a plurality of hard drives, pluggably disposed in the hard drive cage and electrically connected to the hard drive backplane. An extendable/retractable support arm and a wire set arranged inside it maintain an electrical connection of the hard drive cage.