Reply #3: Jumpers are

[democracyindanger]

little plastic/metal blocks that fit over the pins on the back of your drive. You don't have to install them--they're already there. Depending on where you physically place the jumpers determines the slave/master status of the drive. As installed, your old drive was set to 'master' via the jumpers.

Your PC probably didn't work with both drives because of the conflict created by hooking up two master drives. The BIOS was probably confused, so it did nothing.

Here's a page I found with pictures. It looks pretty helpful.

http://www.harddriveupgrade.com/jumper_pins.shtml