Abstract: A computer system with densely-mounted components and effective cooling is provided. A hard drive mounting structure for “hot swap” hard drives utilizes a hard drive assembly in which a hard drive is mounted between a pair of parallel rails connected by a retaining portion. The rails provide a precise mechanism for loading and unloading the “hot swap” drive, without increasing the overall height of each hard drive assembly. A handle with double-cam actuation is used during insertion and removal of the hard drive assembly. In accordance with the present invention, two half-height hard drives may be stacked in a server mountable in a 2U rack. A tool-less lock is provided for releasably securing expansion cards to the computer case without the use of screws.