I replaced the motherboard and processor on a 3 year old HP. I have double-checked all connections, but when I power up, it will not detect the harddrive, gives the message "No Bootable Device"
Your HP has an OEM OS (As d all OEM mfg - Dell, Acer, etc) Their OS is tied to the original motherboard, unless you use the exact same model, The OS will not boot. You need to by a new OS