As far as I understand your old motherboard was addressing your SATA drive in a PATA emulation mode. In this case your hard disk should have a name such as /dev/hdx. But since your current mobo correctly detects it as a SATA hard disk and assigns it the correct name od /dev/sdx.
What you should do is:
- Reboot from the CD in rescue mode.
- Mount your boot volume
- edit your /boot/grub/device.map. It should look like this:
# this device map was generated by anaconda
(hd0) /dev/hda
change /dev/hda to /dev/sda
- save exit and reboot.