One bad lead can cause a computer to continue on a cycle or to shutdown or fail to detect/ boot up a computer hard drive

Test all leads that attach to your hard drive including electrical extensions,IDE,SATA

the leads from your "((MOTHERBOARD TO YOUR HARD DRIVE))" make sure they have a secure connection and are not faulty or just replace them they could be faulty
make sure all leads that are attached to your drives dvd\cd 3 1/2 inch floppy have secure connections and are not faulty even the electrical extensions or just replace them they could be faulty a computer needs its connections to continue its cycle and have an end so any faulty leads will end up with a computer error even something as small as a faulty electrical extension or fan lead can cause this problem

follow the troubleshooting procedure a computer needs all of the data and electrical current to travel through every device and to have an end to be able work properly
a motherboard and a hard drive any leads between them will fail before your motherboard or your hard drive if its a flat ribbon 40 pin type IDE replace it this will be the first to fail check all electrical extensions make sure they are securely seated even the cd/dvd floppy drives need to have current go through make sure these drives are working
hope this helps