Why won't my Xserve G5 recognise my harddrive?

Hi, I recently got an Xserve G5. I put a 2.5 hdd in the sled and popped it in. I booted the Leopard DVD, and Disk Utility will not recognise the drive. What am I doing wrong? Do I seriously need a full-sized drive? Do I need a jumper block? Do I need a blank sled in the other 2 slots? Also, the sled only lights up in the middle drive (the top green light only.)