In general, a disk drive is fixed in a disk drive holding module, and then the disk drive holding module is placed in an insertion space of a chassis. After the disk drive in the holding module is electrically connected to a back plate, installation of the disk drive is completed. However, there are various server chassis with different specifications, and there are also a variety of detaching structures available for detaching the disk drive holding module from the chassis. In particular, the conventional detaching structures often have a large size (i.e. they often protrude from the disk drive holding module), so they need a larger space for operation. As a result, it is difficult to reduce the size of the server chassis and to receive more disk drives in a limited space.