A basic principle of SCSI says that a SCSI bus requires correct electrical termination at both ends in order to function properly. Proper termination ensures that the signal travelling down the SCSI bus doesn't reflect back, a situation that cause a variety of problems including "ghosted" SCSI devices, data errors, and other anomolies. Unfortunately, termination is implemented differently from SCSI device to SCSI device (some use jumpers, some use removable terminating resistor packs, others use combinations of the two), and this can cause some headaches when configuring multiple devices on a SCSI bus. In addition, there are two types of SCSI termination: active and passive. It is important that all devices on the SCSI be configured to use the same type of termination. Of the two types, active termination is by far the better and more common choice.
Active SCSI termination requires three elements, (at least) one termination power source, and two terminating SCSI devices - one at each end of the SCSI bus. With active termination, one or more devices on the SCSI bus provide termination power to the bus, and devices at each end are configured with termination enabled. In most cases, the SCSI host adapter itself sits at one end of the SCSI bus and provides one side of the termination. The last SCSI device on the cable (whether internal or external) must also be terminated. In situations where both internal and external SCSI devices are attached to the host adapter, the SCSI host adapter must be configured with termination off (the most recent SCSI host adapters will do this automatically, but you should always double-check the setting just to be sure).
Sounds simple, right? It can be, but sometimes you'll discover that not all devices are created equal in regards to termination. Some devices (generally the more expensive, higher quality ones) make good bus terminators and some do not. When they don't, it's generally because they use lesser quality components or different terminators with inconsistent Ohm ratings (SCSI-1 devices use 220-330 Ohm terminators, SCSI-2 uses 90-110 Ohm terminators, and SCSI-3 uses 90-95 Ohm terminators). As a result, it is possible that you could still experience problems even when the bus appears to be correctly terminated. Termination problems might manifest themselves in a number of ways, includng duplicate or "ghosted" devices, missing devices, a hang at boot when the SCSI card initializes, or problems accessing SCSI devices attached to the bus. If a particular configuration doesn't work, try re-terminating the bus with a different device (or devices) at the end(s). Also, it's probably a good idea to have more than one SCSI device providing termination power to the bus when many devices are present. A good rule of thumb is about one term power source for every 2-3 devices on the bus. You don't have to worry about this causing overvoltage problems either, since all SCSI devices have components to protect against this.
One way to circumvent SCSI termination hassles altogether is to use a separate external active terminator for your SCSI bus. Using one of these means you won't need to rely on the devices themselves to provide termination to the SCSI bus, and can be assured that the proper termination is being supplied. One vendor of such products, Granite Digital (Union City, CA), makes both active terminators as well as special diagnostics cables that have LEDs telling you exactly what's going on with the SCSI bus. These products really take the guesswork out of SCSI troubleshooting, and I highly recommended them.