Why is this Win7 Ultimate x86 PC showing an error 80-pin Connector not found and boots only into win7 RE recovery mode?

The PATA interface was originally designed as a 40-pin system and still is. The ribbon cables were originally 40-wire cables. A special type of cable with 80 wires was then created to improve data throughput rates. Note that the number of pins remains 40, with a special type of ribbon connector connecting ground to 40 wires internally. The ground connections are placed between each of the data carriers to reduce cross-talk and allow up to 3 times faster operation. ATA33 is supported by 40-wire connection, ATA66 & ATA100 require 80-wire. Motherboards can check the speed of the data throughput to determine if a 80-pin cable is being used. Some BIOS setup options allow for this feature to be turned off. The HDD or DVD must support ATA66 or higher to work at the higher speed of a 80-wire cable.If you connect a slow hard disk or DVD drive on the same ribbon as a fast hard disk then the system will default to the slower speed. You also need to make sure the master and slave are connected to the correct connectors on the cable.
Firstly try each drive separately to see if the 80-pin error goes away.
Rather than master/slave on one cable, try each hard drive as a master on separate cables (IDE1 & IDE2).
Secondly, the boot partition may have corrupt data preventing boot. You should check both hard drives for errors by connecting to another PC if possible. This would also allow you to back up any important files so that you can re-install the operating system if required.