FIXED MAGNETIC RECORDING DISKS
Fixed disks, or hard disks as they're more commonly called, are expensive, rigid, semi-portable, magnetic storage mediums. They have the following characteristics:
Hard Disk Construction
Most hard disks are made of aluminum platters coated on both sides with either iron oxide or thin-film metal magnetic coatings. The first type, iron oxide, is the most common (you can recognize this coating by its rust color). This is the same oxide coating that's used on magnetic tape. The second type of coating, thin-film metal, is the newer and better of the two. This coating is a microscopic layer of metal that's bonded to the aluminum platter. You can recognize it by its shiny silver color. Thin-film metal-coated hard disks are becoming more and more popular because they allow more data to be stored in less space.
Hard disks can hold a lot of data, the smallest disk being 10,000,000 bytes, and the largest being about 2,500,000,000 bytes (and they're working on larger ones).
Hard disk platters come in many sizes, ranging from 14" to 2".
The most common sizes are 3-1/2", 5-1/4" and 14". The first two sizes are usually used with smaller personal computers. The 14" size is usually used with the larger mini and mainframe computers.
Most hard disk drives use more than one hard disk platter to store data. These are called disk packs. Some hard disk drives use removable hard disk platters. These can use just one platter, or they can use disk packs containing many platters. Most of the multi-platter removable hard disk drives in use today use 14" hard disk platters. Figure 8-4 shows a hard disk-pack.
Figure 8-4. - Magnetic hard disk pack.
Hard Disk Data Organization
Data is stored on a hard disk the same way it's stored on a floppy disk, in circular tracks. The total number of tracks on a hard disk is set, just like floppy disk, by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the hard disk platter.
A computer places data on a hard disk using one of two methods, either (1) the cylinder method, or (2) the sector method. The manufacturer of the hard disk drive decides which method to use.
THE CYLINDER METHOD. - This method uses a cylinder as the basic reference for placing data on a hard disk. Look at figure 8-5 view A. This is a picture of a disk pack containing six hard disk platters. Notice that this particular disk drive uses only 10 out of the 12 available recording surfaces. If you imagine that you're looking down through the disk pack from above, the tracks with the same number on each of the 10 recording surfaces will line up. Put together, these tracks make up a cylinder. Each of these 10 tracks with the same number, one on each recording surface, can be read from and written to by one of the disk drive's 10 read/write magnetic heads that are positioned by the five access arms.
Figure 8-5. - Cylinder and sector method of organizing data on a hard disk pack.
So, to locate a place to store data using the cylinder method, a computer must specify the cylinder number, the recording surface number, and the record number. Figure 8-5 view A shows record number 1 stored on cylinder 25 of recording surface number 6.
Special data is stored on each track to tell the computer where the start of a track is.
THE SECTOR METHOD. - Although we talked about this method earlier under the heading "Floppy Disk Data Organization," we need to repeat it here as it also applies to hard disks.
The sector method of organizing data on a hard disk is actually a variation of the cylinder method. As you already know, the sector method slices up a hard disk into pie-shaped slices (just like floppy disks). The total number of slices is set by the hard disk drive manufacturer. Figure 8-5 view B shows an example of the sector method.
Unlike a floppy disk drive, which locates a place on the disk using the surface number, track number, and sector number, a hard disk drive locates a place on the disk by using the surface number, cylinder number, and sector number. This is true even if the hard disk has only one platter. That's because both surfaces of that one platter still form a cylinder.
Hard Disk Handling, Storage, and Shipping
Hard disks hold a lot more data than floppy disks; even the lowest capacity hard disk can hold 5,000 pages of data! That's why it's important to handle, store, and ship hard disks properly. If you think 180 pages of data is a lot to retype, just think of retyping 5,000 pages!
Hard disk drives spin at a very high speed of about 3600 RPM. It is extremely important that nothing gets between the head and the recording surface. If it does, you can lose data and you can damage both the magnetic head and the disk's recording surface.
Most hard disk failures involve a head-crash. It's the worst thing that can happen to a hard disk. A head-crash is the result of the disk drive's magnetic heads crashing into the recording surface and grinding into the hard disk platter.
Figure 8-6 shows a good hard disk platter and a bad hard disk platter that was the victim of a head-crash.
Figure 8-6. - Example of a hard disk crash.
You must handle, store, and ship hard disks with extreme care if you want them to stay in good condition. Here are some specific precautions you should take:
Hard Disk Erasing
There are two ways to erase a hard disk: (1) degauss it and then reformat it, or (2) just reformat it. As you might guess, the first method can only be used for removable hard disk platters. The second method (reformatting) is the most common. If you must degauss a removable hard disk, the process is the same as degaussing magnetic tape. Refer back to chapter 2 for the details on this.
If the hard disks were used to store classified information or unclassified but sensitive information, you can't de-classify the hard disks by erasing them. This is because with the right equipment and software, the data that was on the disk can be reconstructed.
If you can't re-use the hard disks to store other classified data, you must sanitize or destroy them, using the procedures in OPNAVINST 5510.1.
Q.11 What are the three most common sizes of hard disk platters?