A computer includes a cabinet, a plurality of hard disk drives (HDDs), and a plurality of HDD trays. The cabinet defines a plurality of groups of screw holes therethrough. Each of the HDDs defines a group of screw holes in the bottom surface thereof respectively. Each of the HDD trays includes a loading bracket and a plurality of supporters, and the supporters extending outward from the loading bracket; the loading bracket defines a plurality of arrays of screw holes, each array of screw holes having a unique size to allow a corresponding size of HDD to be secured thereon, the supporters define a group of screw holes corresponding to the screw holes defined on the cabinet.