The invention proposes a storage apparatus and conversion board that can increase the number of hard disk drive heads in a given, limited space for installation of disk units in the storage apparatus, and can consequently improve data read/write performance.A storage apparatus has a plurality of first disk units of a specific size, each removable, and a controller that controls data read/write from/to the first disk units, and the storage apparatus includes: external connectors, each provided in corresponding positions where the first disk units are installed, and physically and electrically connected with the first disk units installed in a specific state; and a removable conversion unit provided in one or more of the external connectors, and used for installing, in the position where one or more of the first disk units are to be installed, a larger number of second disk units, each smaller in physical size, than the one or more first disk units.