The EIDE interface is the main storage connection in most PCs, not just for hard drives and CD-ROM drives but for CD-RW and removable-media drives as well. A poor configuration can hurt performance, so the more devices you hook up, the more important managing the multiple connections becomes.

All standard desktop PCs have two EIDE channels (primary and secondary), each of which can handle two drives, designated as master and slave. If four drives aren't enough, you can purchase cards that add an extra pair of EIDE channels for $50 to $75. These cards also support fast Ultra DMA/66 (aka ATA/66) hard drives, which don't work to their full potential in PCs more than a couple years old. (Other EIDE peripherals, such as tape drives and CD-RW drives, don't need or use the extra speed of Ultra DMA/66.)

If your PC is more than three or four years old, it may have problems recognizing some newer EIDE drives, so you may need to update your PC's BIOS. Some PC makers let you do that by downloading a file from their Web site. Otherwise, you'll have to buy an upgrade chip (about $50).

As always, make sure you have a full backup of your PC before you do any major upgrade work. If you'll be installing a tape backup drive, copy your most important data to floppy disks until you're ready to use the drive.

Turn off your PC, unplug it, and remove the cover. Depending on the size of your new drive, find a free small (3.5-inch) or large (5.25-inch) drive bay. If you're installing a peripheral (such as a CD-RW drive) that requires front-panel access, remove the bay's cover from the front panel of your PC's case. If you have a small drive and don't have an extra 3.5-inch bay, don't worry: Most drives come with an adapter that allows them to be mounted in a 5.25-inch bay. If your drive doesn't, you can obtain an adapter at extra cost.

Before you go any further, make sure that the wide data cable has a second connector for hooking up the new drive and that this connector can reach the drive in the bay you're planning to use. Also make sure that you have a free red, yellow, and black power connector. If not, you'll need to buy another ribbon cable or a Y-adapter that creates two power connectors out of one.

If, as is normally the case, you're installing your new drive as the second drive on an EIDE channel, make sure the jumper on the drive is set to slave. If it's not clearly marked on the drive, consult the manual.

If you're installing a second drive on the primary EIDE channel, to go along with your PC's primary hard drive, you may need to change the jumper on your main hard drive. Some hard drives use different settings, depending on whether they're the only drive on an EIDE channel.

Mount the drive in the bay, using the screws that came with it. (In some cases, you may need special brackets. Extra brackets usually come with your PC. If not, contact your PC maker to obtain extras.)

Attach the free connector on the wide data cable to your drive, making sure that pin 1 (the red wire) on the cable goes to pin 1 on the drive. There's usually a key on the cable that allows you to connect it only one way. If not, pin 1 is usually marked on the connector on the drive, though you might have to look closely to find it. Make sure that the other end of the cable is securely attached to the EIDE connector on the motherboard.

Plug in a power connector to the new drive.

Enter your PC's setup program. Details vary by manufacturer, but you'll often see a message such as "Press the DEL key to enter setup" as your PC starts up. Make sure the BIOS setting for each of the drives is set to AUTO, which should automatically detect the new drive and set up the correct parameters.

Save the setup settings and reboot your PC. If your new drive requires special software, install it, and test the drive.

If Windows doesn't recognize the new drive, double-check all your connections. If that doesn't work, it's time to call tech support.

Benefits: Bigger and faster hard drives, removable-media drives, and tape drives; faster CD-ROM and CD-RW drives

Costs: Hard drives, $125A-$250; CD-ROM drives, $50A-$130; CD-RW drives, $200A-$300; removable-media drives, $100A-$150; tape drives, $150A-$300

Expertise level: Intermediate

Time required: 1A-2 hours

Tools required: Phillips screwdriver, needle-nosed pliers, antistatic wrist strap

Additional EIDE channels: Promise Technology

A BIOS chip:Unicore