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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).