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A bit more on SCSI, to build on ivan37's wu above.

You may have one or more SCSI ports on your motherboard, or you may be using one or more SCSI adaptors (usually a PCI card). If you're using a card, the one that's plugged in closest to the power supply will probably be card 1, and so on accross the motherboard.

The machine will try to boot off SCSI devices in order, so your boot drive should be on card one, with a low SCSI ID, such as 0. ID 7 will be occupied by the adaptor itself. Beyond this, SCSI IDs are largely unimportant. You can attach as many devices as will fit (ie. 13). If you have two devices which you know will talk to each other a lot, such as a CD-ROM and a CD-RW, try to put them on the same SCSI bus.

A SCSI bus is made up of the adaptor, the cable and any devices. The bus needs to be terminated, either by setting the termination jumper on a device attached to the last connector on the cable (ie. furthest from the adaptor), or by attaching a dongle-type terminator. There should be a label on the disk telling you which one is the termination jumper.

The connectors, cable and devices can be one of two types (your adaptor or motherboard may even have both types on it) - these are Narrow SCSI and Wide SCSI. Narrow SCSI connectors are wider then Wide SCSI connectors - the name comes from the number of pins (Narrow = 50 pins, wide = 68 pins). You can get adaptors which allow you to plug a wide SCSI connector into a Narrow SCSI device, but not vice-versa. Check wangmu's wu for more on eclectic SCSI connectors