I own a DELL Optiplex 745 Mini Tower with 160 Gb SATA Hard drive. I attempted to add a second 160Gb SATA Hard drive. I connected the extra power supply to the second drive. I then connected a SATA data cable to the hard drive with the other end plugged in to a slot on the motherboard. I then booted my computer. However, the computer cannot see the second hard drive. Is there any other thing which I should have done? Thanks for your advice.

You probably have to turn it on in the BIOS. I believe you have to press F2 when you see the Dell logo to get into it, while booting. I'm not sure about that exact model, but I *think* that somewhere there is a list of SATA-0, SATA-1, SATA-2.. you highlight the one you want and press enter, the cursor jumps to the other side of the screen, and you can change it to on or off. Just make sure you have both of them turned on for where the hard drives are, and save and exit.

Should 1 HD be master, and 2 HD be slave -- jumpered ?
That would be if it were an IDE drive. You can only have 1 SATA device on any particular SATA port, so you don't need any special settings on them like Master and Slave to get them to work together.