i dont know corectly but i know some thing about it
With hard disk drives, both speed and size matters. In terms of the brand, I would personally stick with the leading brand names like IBM, Seagate, Western Digital, or Maxtor. With hard disk drives there are basically five different types, EIDE, SCSI, Firewire, Serial ATA, and USB.
For SCSI, Firewire, and Serial ATA drives, you may need a separate interface card and the hard drives themselves are more expensive, but faster than EIDE drives.
Within EIDE you have three different types, regular EIDE, Ultra DMA and Ultra ATA. The difference is that Ultra DMA and Ultra ATA have higher speed in terms of disk access and data transfer since the hard disk runs faster (DMA 5400 RPM, ATA 7200 RPM and higher) and because of a special interface the data transfer rate is higher (66 MB/sec, 100 MB/sec, or 133 MB/sec).
Serial ATA drives are the newest type of hard drive and promise transfer rates of 150 MB/sec or more when used with a native serial bus controller.
The access to data on the disk (seek time) is very important for the overall performance of the computer. Data transfer rate is important if you are using big files like big databases or video files. Hard disk drives are relatively cheap.
Recommended hard disk: 40 - 200 GB, 7-9 ms seek time, 7500 RPM, ATA 100 or 133. See our articles Installing a Second Hard Drive, and our InfoHQ Tech Watch Newsletter for more information about hard drives.
Hard disk drives are accessed over one of a number of bus types, including ATA (IDE, EIDE), Serial ATA (SATA), SCSI, SAS, and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to busses that they cannot communicate with natively, such as IEEE 1394 and USB.
Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by fifty percent, to 7.5 megabits per second; it also increased disk capacity by fifty percent.
Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)
Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.
SCSI originally had just one speed, 5 MHz (for a maximum data rate of 5 megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early IBM PC compatibles and early Apple Macintoshes.
ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run in a master/slave setup (two disks on the same cable). This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).
Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.
FireWire/IEEE 1394 and USB(1.0/2.0) hard disks are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself.
[edit] Disk families used in personal computers
Notable disk families include:
Bit Serial Interfaces - These families connected to a hard disk controller with three cables, one for data, one for control and one for power. The hard disk controller provided significant functions such as serial to parallel conversion, data separation and track formating, and required matching to the drive in order to assure reliability.
ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
ST412 was available in either MFM or RLL (Run Length Limited) variants.
ESDI (Enhanced Small Disk Interface) was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
Word Serial Interfaces These families connect to a host bus adapter (today typically integrated into the "North Bridge") with two cables, one for data/control and one for power. The earliest versions of these interfaces typically had a 16 bit parallel data transfer to/from the drive and there are 8 and 32 bit variants. Modern versions have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor hard disk controller.
Integrated Drive Electronics (IDE) was later renamed to ATA, and then PATA. The name comes from the way early families had the hard disk controller external to the disk. Moving the hard disk controller from the interface card to the disk helped to standardize interfaces, including reducing the cost and complexity. In 2005/2006 parlance, the 40 pin IDE/ATA is called "PATA" or parallel ATA, which means that there are 16 bits of data transferred in parallel at a time on the data cable. The data cable was originally 40 conductor, but UDMA modes from the later disks requires using an 80 conductor cable (note that the 80 conductor cable still uses a 40 position connector.) The interface changed from 40 pins to 39 pin. The missing pin acts as a key to prevent incorrect insertion of the connector, a common cause of disk and controller damage.
EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of DMA to transfer data between the disk and the computer, an improvement later adopted by the official ATA standards. DMA is used to transfer data without the CPU or program being responsible to transfer every word. That leaves the CPU/program/operating system to do other tasks while the data transfer occurs.
SCSI (Small Computer System Interface) was an early competitor with ESDI, originally named SASI for Shugart Associates. SCSI disks were standard on servers, workstations, and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only. The length limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended data transmission, but server class SCSI could use differential transmission, and then Fibre Channel (FC) interface, and then more specifically the Fibre Channel Arbitrated Loop (FC-AL), connected SCSI hard disks using fibre optics. FC-AL is the cornerstone of storage area networks, although other protocols like iSCSI and ATA over Ethernet have been developed as well.
SATA (Serial ATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. The same differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
SAS (Serial Attached SCSI). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands for interacting with SAS
Acronym Meaning Description
SASI Shugart Associates System Interface Predecessor to SCSI
SCSI Small Computer System Interface Bus oriented that handles concurrent operations.
ST-506 Seagate interface
ST-412 Seagate interface (minor improvement over ST-506)
ESDI Enhanced Small Disk Interface Faster and more integrated than ST-412/506, but still backwards compatible
ATA Advanced Technology Attachment Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations