The Small Computer Serial Interface once formed the principal connection protocol used in Apple's Macintosh computers and on drive arrays. SCSI remains a viable choice for some specialized storage implementations, but it lacks the drive capacities, inexpensive pricing, simple connectivity and wide acceptance to make it competitive with SATA on a broad scale. SCSI drives require special attention to their preparation, cabling and formatting to assure proper function when you put them in service.
Incorrect Termination
A SCSI bus requires electrical termination on the last device attached to it. If you installed your hard drive inside your computer and hooked it up to the last connector on the ribbon cable connected to a SCSI bus -- built-in to your system's motherboard or supplied through a PCI host adapter. Add the proper jumpers to the drive's termination block. Jumpers look like hollow black rectangles and come in more than one size. The dimensions you need depend on the design of the jumper block on your drive. Adding termination jumpers to a SCSI drive in the middle of a chain can interrupt the proper function of the device and any other mechanisms on the bus, unless your cables run very long. Some host adapters enable the devices attached to them to terminate without any jumpers. Incorrect termination can make a drive inaccessible or cause it to appear more than once on your system.
Improper IDs
Each drive on a SCSI bus uses a numeric ID that enables your computer to recognize and address it uniquely. You can assign IDs ranging from zero through six on narrow SCSI buses and from zero through 14 on wide SCSI buses. On a system with more than one bus, you typically can use the same ID number on each, provided that the bus contains no other device with that ID. For example, each of two buses can include a device set to ID 6, but you can't assign ID 6 to two devices attached to the same bus. If you set a drive's SCSI ID incorrectly, it may not become visible to your operating system, rendering it impossible to format. Internal drives use the same kinds of jumpers to assign their IDs as they do to apply termination, with jumper position and orientation determining the ID assignment. Many drives include a block diagram on their labels to aid you in applying jumpers correctly.
Wrong Formatting Utility
Some systems require the use of aftermarket formatting tools to address hard drives supplied without a special chip that enables the computer to access these mechanisms. If you're trying to install a SCSI drive into an older Macintosh computer, you may encounter this very scenario (see Resources). In this case, the formatting tools supplied with your operating system report errors when you attempt to use them to prepare your drive. Once you use a third-party formatter on a mechanism, you must continue to use it, or use it to erase the drive and remove the formatting, so you can reformat the mechanism with another utility.
Other Considerations
If you've resolved the termination, ID and software conundrums that can prevent a SCSI mechanism from accepting formatting instructions, test the drive to assure that your problem doesn't stem from faulty hardware. You also may be attempting to use a recently manufactured SCSI drive in a computer that can't support new versions of the SCSI protocol. Some systems only can see limited amounts of the capacity of large drives when you attach them to an internal bus. When you place a large drive in an external case, you may gain access to its full capacity, but now you face the need to verify that the case itself functions correctly. If its power supply fails to function correctly, your computer no longer can address the drive mechanism.
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