Since I don't know what you tried I'll have to guess:
From my experience most Board systems have more than one limitation (by dimension and by size).
Just check if one of those conditions isn't met for that particular board.
System Manufacturer/Model Number HP m8000n OS Windows 7 Ultimate x86 CPU DualCore AMD Athlon 64 X2, 2600 MHz 5200+ Motherboard Asus M2N68-LA (Narra) Memory Samsung 2GB DDR2 Graphics Card Onboard NVIDIA GeForce 6150SE nForce 430 Sound Card Onboard nVIDIA nForce 6100-430 (MCP61P) Monitor(s) Displays Westinghouse 19" LED Screen Resolution 1280x1024
Keyboard Logitech Classic KB 200 Mouse Standard HP opticle USB mouse PSU Stock (HP) Case Stock (HP) Cooling Stock Hard Drives SATA II Seagate Barracuda 500GB
USB II WD Elements 500GB
USB II WD My Book 1TB
USB II WD My Book 2TB