The current hard drive is a 200 GB, 5400 RPM Toshiba. Wish to replace it with a Western Digital Black 320 GB, 7400 RPM. First, mounted the WD HDD on an external case and cloned the partition where the OS is located. Next I changed the priority order in the BIOS to boot from the external drive to confirm the cloning was successful. However, when I installed the new WD hard drive into the laptop, the system won't boot it can't find the operating system. Did I miss something? Maybe some one can help? Thanks.