The HP D50 comes in three different cases depending upon your needs.
The Small Form Factor (SFF) units have limited expansion capabilities and is ideally suited as a workstation on a network. It has room for one internal drive (CD, DVD, Tape etc) and some expansion cards. These units can lie flat, or stand vertically in the mount supplied.
The Convertible Mini Tower (CMT) units have room to burn. No problems with expansion. These can operate as desktop units or tower with the 3H drive bay being able to be rotated so that the CD can be horizontal.
The Ultra Slim Desktop (USDT) units are just that. Small, compact and ideal as network workstations where expansion is not required, but personal devices (hard drive, OS, USB etc) are. If you do not want your own hard drive etc, consider an HP EVO Thin client solution to a Terminal server.