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More on Hard Drives

 

Although the the next three images will not really further your knowledge of computers, I thought a few people might be interested in seeing them. You'll have to excuse the quality of the second and third images. The object is almost microscopic. The image immediately below is the magnetic actuator out of the hard drive I showed you on an earlier page of the tutorial. At the far left are the heads. To the far right, you can see the 'voice coil' of the magnetic actuator circuit. There is a strong permanent magnet in the base of the hard drive. When voltage is applied to the voice coil, the electromagnetic field interacts with the field of the permanent magnet and the actuator moves.

And this is a little closer...

The image below is the side of the head assembly that is closest to the platter. On this old drive, when the platters spin down, the head assembly rests on the platter. On newer drives, the heads are moved off of the platter before they spin down. On newer drives, the heads never touch the platters under normal operating conditions.

The following image shows the end of the head assembly. The head is the little shiny spot in the middle of the epoxy (where the wires terminate). Although you can't tell from the picture, the wires (two twisted together) are about 1/2 the size if a human hair. On a hard drive this old, it's possible that the head uses a coil of wire as a magnetic pickup. On newer heads, they use a different (magnetoresistive) technology that allows tighter spacing of the tracks on the disc platter which allows higher capacity. The new drives can get 100GB or more on a single platter. This drive had only 1.6GB per platter.

The following image shows a different hard drive that allows a better view of the platters.

This is a wide shot of the platters, actuator arms and heads.

In this close-up of the drive, you can see how the heads sit on the platters. If this were a working drive and the platters were stopped, the heads would be near the center of the platters (near the spindle) in what's known as the 'landing zone'. The landing zone is an area where there is no data. On newer drives, the heads would be moved off of the platters before the platters stopped. Remember that on new drives, the heads never actually touch the platters. They 'fly' just above them on a cushion of air.

Hard Drive Specifications

Capacity:
The 'capacity' of the hard drive is simply the amount of data it's rated to hold. The actual reported data capacity (reported by your OS) may be somewhat less. That's because the 1k (1000) in our number system (the decimal system - based on the number 10) is slightly different than the 1K in the number system used by computers (binary - based on the number 2). In the decimal system, 1K is equal to 1000. In the binary system, 1K is 1024 (2 raised to the 10th power). At 1K, the difference is only ~2.5%. In the 1M range, the difference is ~5%. In the 1G range, the difference is ~7.4%. This is why a 200GB hard drive will only register as a 186GB drive.

Average Seek Time:
This is the average time that passes from the time the data is requested to the time the data is delivered.

Average Latency:
This is the average time it takes for a point on the platter to come around to the heads. It's half of the time for one rotation. The average latency is determined by the rotational speed alone.

Buffer Size:
The buffer is the on-board memory of the drive. Commands or data can be stored there if the drive is busy when the data requests come in or the motherboard is busy when the data is ready to be returned.

Rotational Speed:
This is the rotational speed of the spindle and platters in the hard drive. It's specified by RPMs (Revolutions Per Minute).

Interface:
There are three common interfaces. The most common in personal computers is the IDE interface. The second most popular is likely the SATA interface. SCSI drives have been around for a long time but they're expensive. SCSI drives are typically used in servers. Very few find their way into home computers.

Form Factor:
The 3.5" form factor is the most common size hard drive.

NCQ:
Native Command Queuing is a technique that a hard drive uses to more efficiently retrieve data. For most hard drives, the data is retrieved in the order it was requested. This can be inefficient if the data is spread out in a way that takes the heads over the same point more than once. NCQ looks at the location of the data, if the last requested data is 'on the way' to earlier requested data, then it will pick up the last requested data before the earlier requested data.

Hard Drive Structure

Until now, we haven't gone into much detail about the way a hard drive is structured. As you know, the hard drive has a series of stacked platters. The platters are essentially set up the same way in all hard drives. There are slight variations but the following is a good general description of the common hard drives used in personal computers.

Tracks, Sectors and Clusters:
On each platter we have a series of concentric circles. They are called 'tracks' and there are thousands of them on the typical hard drive platter (I only show a few on the following diagram for clarity). Each of those tracks is broken up into 'sectors'. They are broken up by imaginary lines that cross from one edge of the platter, through the center of the platter and to the opposite side. You can see the tracks and sectors below. When data is written to the disc it is stored in clusters. Clusters are groups of sectors. If the data won't fit in a single cluster, the computer will try to write the data to adjacent clusters. When it can not (because the disc doesn't have a large enough area with enough adjacent clusters for the file you're saving), it finds other clusters in which to write. When a file is in non-adjacent clusters, the file is fragmented. The file is still perfectly usable but it may take a little longer to retrieve it (although you probably won't notice the few extra milliseconds). Clusters vary in size depending on the size of the drive/partition and the operating system or formatting. With all operating systems, there is a limited number of clusters. If you have a very large drive and an operating system that has a very limited number of clusters, the clusters may be quite large. Since only one file can be written to a cluster, large clusters can lead to inefficient use of disk space. Luckily, today's operating systems have a sufficient number of clusters to allow small clusters even with the largest of the available hard drives.

Cylinders:
When writing data to the hard drive, you may think that the data would be written until one side of the platter was full then it would move to the next platter or to the opposite side of the same platter (whichever was next). After all, we've all seen phonographs play from the outside in and CDs and DVDs play from the inside out. It seems only natural that the hard drive would operate in a similar fashion. That's not the case. A hard drive will write to a series of sectors (along a single track) and then when that track is full (if it has more data to write), it moves to the next track in the same 'cylinder'. As you can see below, we have a stack of six platters. The tracks are all aligned vertically. Let's imagine that you had a single track on each side of each platter and that each track was one inch high and the cylinder walls were only as thick as a single track. If you laid all of the platters on top of one another, the one inch high tracks would form a thin-walled cylinder (sort of). In the following diagram, you see how the 'cylinders' pass through the corresponding tracks of all of the platters. The number of cylinders is equal to the number of tracks on one side of a single platter. If you have 20,000 tracks on a single side of a single platter and you have 3 platters, you only have 20,000 cylinders.

Other Information on the Hard Drive:
Hard drives have to store information about the way the information is stored/arranged on the platters. This is done in at least 2 different ways. One way to do it is to have one entire platter dedicated to such information. The other way is to have the information stored in fixed areas of the data discs.

SATA Hard Drives

Earlier in the tutorial, i showed you a picture of an older hard drive with the cover removed. It's heads and actuator arm are shown at the top of this page. The hard drive you see below is a newer hard drive. It has some things in common with older drives and there are some new features. The drive below is Western Digital Raptor SATA (Serial ATA) drive. It's designed to be faster than the average drive but there are tradeoffs in the design. It approaches the speed of some SCSI drives but costs significantly less than a SCSI drive. These high-performance will typically run hotter than a standard drive. The average drive spins its platters at 7200RPMs. The drive below operates at 10,000RPMs. Keeping the platters spinning that fast simply takes more energy and causes the drive to run hotter. The upside to the high rotational rate is that the information comes around to the heads more often than with the 7200RPM drives (lower average latency).

Note:
When using any high performance drive, I strongly recommend that you install a fan in front of the hard drive bay. Most computer cases provide some sort of fan mounting area for 80mm fans in front of the bay. Keeping air moving over the drive (even a small amount of air flow) will significantly reduce the operating temperature of the drive. The fan in this location also brings fresh air into the computer case. This is important if the case doesn't have a side mounted intake fan.

If you look at the connectors on the drive above, you can see that the connectors are significantly different than the connectors on the rear of the CD-ROM drive that was shown on the 'storage' devices page. The drive above uses a different data cable and has the option to use the new style power cable or the standard 4-pin Molex. The image below is the bottom of the SATA drive and gives you a somewhat better view of the connectors.

The image below is the SATA data cable connector. As you can see, it's quite a bit smaller than the 40 pin IDE cable. This makes it easier to route the cables, reduces clutter and allows for better air flow through the case. They also have a longer maximum length than IDE cables (which are limited to 18 inches).

These are the SATA power connectors. There's nothing special about them really. If you buy a SATA drive and your power supply doesn't have SATA power connectors, the Molex to SATA adapters are readily available.

Partitioning Hard Drives

Do not try to change, add or remove the partition on any drive unless you know precisely what you're doing. The slightest mistake may leave you with an inoperable computer and all loss of data on your hard drive. The following image shows the disk partition manager for Windows. It shows all of the partitions on all of the hard drives. As I mentioned earlier, notice that the main drive (the 40GB Samsung drive that you saw in the 'inside the computer's case' page) has 2 partitions. The small laptop drive in the external USB drive case has only one partition.

On most computers, the entire hard drive space is one partition (sort of like a large building with no internal walls to divide the living space into individual dwellings/apartments). When you set up multiple partitions, you are essentially putting up walls in that building. There have been times that the Windows OS quit working properly for one reason or another (virus, disc error...). Sometimes, the problem is severe enough that you have to reformat (erase and start anew) the entire partition where the OS resides. If you have to wipe the entire partition and you have all of your photos and music on the same partition as the OS, you lose everything. Many times, if your files are on a separate partition, they are safe. I generally setup a 15GB partition for the operating system and programs and leave the rest for my files (in a second partition). Also, if you like to keep your hard drive defragmented, it takes less time to defrag individual partitions than it takes to do the entire drive. If you import/export a lot of large files (like video files), you will badly fragment your hard drive. Doing the video editing on a partition other than the one where your operating system is, will help prevent the OS partition from getting fragmented.

Defragmenting:
As we mentioned earlier, sometimes there isn't enough contiguous space (adjacent sectors) to write an entire file. When the file is in non-adjacent sectors, the file is fragmented. Defragmenting moves the files around so that as many as possible are in adjacent sectors.

Formatting:
Formatting essentially erases all the information from a partition and readies it for data.

NTFS/FAT32:
When you set up a new drive, you have to format it to comply with one of several file systems. The most common are the NTFS (New Technologies File System -- from Windows NT) and the FAT32 (File Allocation Table - 32bit). XP installations default to NTFS but you can select FAT32. If you're using older software or setting up a dual boot system with Win98 or the like, you need to use the FAT32 system. At one time I preferred the FAT32 file system because it was easier to boot to a floppy disc and solve problems with Windows. Now that everyone is using Windows XP, there is little advantage to using FAT32.

Setting Hard Drive Jumpers

On IDE drives (the ones that use the 40-pin connectors), there are jumpers on the back of the drive. These jumpers tell the motherboard which drive is going to be the master drive (the master drive holds the operating system) and which is going to be the slave drive. As you can see in the image below, there is a red jumper connected across one pair of pins. Beside the red jumper, there are two more pairs of pins.

If you look at the next image, you can see some markings on the bottom of the drive. 'MA' denotes 'master'. SL is slave and CS is cable select. This drive is set to CS (there's a black jumper in this image). If you're using a 40-pin, 40 conductor cable, you must set the jumpers properly for the master and slave drives. In most simple systems, the hard drive is the master and the CD-ROM drive is the slave (if both drives are on the same cable). If you have a 40-pin, 80 conductor cable, you can set both drives to CS and the drive plugged into the end connector on the cable is the master. The drive plugged into the middle connector on the 80 conductor cable is the slave.

In this image, you can see an 80 conductor cable. On the left is the motherboard connector. To the far right (black) is the master drive connector. In the center (but closer to the master connector than the MB connector) is the slave connector. Sometimes, you will have only one drive on a cable. If that's the case, use the connector on the end of the cable (some systems are sensitive to reflections along the cable and will cause errors). If you have a 40 conductor cable, set the single drive as a master.

Jumpers on SATA Drives:
The drive above was an IDE drive (it may also be referred to as a P-ATA drive or simply an ATA drive). SATA drives also have jumpers but they do not serve the same purpose. For the most part, you can simply leave SATA jumpers as they are from the factory. The hard disc controller or the motherboard will make the proper distinctions between the drives.

 

 
 
 
 
 
 

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