A holding device includes a loading frame and two mounting plates. The loading frame includes a base portion for loading an HDD and a pair of sidewalls perpendicularly extending from the base portion. The sidewalls are parallel to each other. Each sidewall includes two sliding ribs substantially perpendicularly extending from a surface thereof away from the other sidewall. The two mounting plates are opposite to each other. Each mounting plate includes two guiding rails substantially perpendicularly extending from a surface thereof toward the other mounting plate. The two sliding ribs of each sidewall are configured to be slid into the two guiding rails of a corresponding mounting plate.