ESX is the lite version of ESX that is free. It is a bare metal install with a minimal install profile and console, designed for small footprint and OEM deployment. As a result it has a limited HCL, and less install flexibility (partial SATA, and most SCSI, but no IDE, and limited compatibility on older server platforms)

The ALT F1 switch, during the install process, will enable some work arounds. See this article from VMware; Install ESXi 3.5 to an IDE drive