The hard disk is a type of secondary storage on youe system. All the program data are stored on the hard disk, & when needed will be transferred to the main memory for execution. As the cost per MB of main memory(using semiconductors) is much more than that of the secondary memory(using magnetic materials), you have to have a hard disk for storage. The performance of the hard disk plays an important part in the overall performance of the system. How quickly is your HDD able to transfer data to & from RAM(transfer rate)? How quickly is your HDD able to access a particular file on it(access time)? Is it cost effective? Finally, how reliable is your HDD?

All the above questions can be answered if we take a look at the two technologies that exist. They are the IDE & SCSI. IDE(Integrated Drive Electronics) is the cheaper of the two, and also the easier to configure. SCSI stands for Small Computers Standard Interface. It was developed to be a cost effective alternative to the high speed tech. that exists in mainframes. But its cost is so high, unless you are buying a server it would entail unnecessary high expenditure. But then, IDE drives are now coming of age, with speeds comparable to SCSI.

The feature to look for if buying an IDE drive is UDMA(Ultra Direct Memory Access). Though most of the drives shipping today are UDMA enabled, you can get those without UDMA support. UDMA has a peak transfer rate of 33MB/s, while PIO mode 4(EIDE-Enhanced IDE) supports upto 17MB/s. Even though the difference might look large, in real world situations, it is the sustained transfer rate that plays a major role(as a matter of fact my JTS 2.1GB champion Ultra ATA(PIO mode 4) drive outperforms my cousin's Western Digital 2.1GB UDMA 2 drive!). But it is a matter of assurance that you have a drive that will provide you with the required bandwidth when necessary. Also look for a drive which has a low access time.

If you are in for a SCSI drive, you can get plain SCSI, Ultra SCSI, Ultra wide SCSI, Ultra wide SCSI 2,...
All the above vary accordingly in their speed. The main advantage of using a SCSI interface, is that you can daisy chain upto 20 SCSI devices to the controller. But the cost factor is a major inhibition to its success in the home pc market.

Which is best?
If you really want to set up a server type system & have lots of moolah, go for SCSI. Otherwise there is no reason not to buy a EIDE drive, as with UDMA, they are approaching the speed of SCSI.

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