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[ The PC Guide | Systems and Components Reference Guide | Floppy Disk Drives | Floppy Disk Media and Low-Level Data Structures ]

Low-Level and High-Level Formatting

There are two steps involved in formatting magnetic media such as floppy disks and hard disks. The first step involves the creation of the actual structures on the surface of the media that are used to hold the data. This means recording the tracks and marking the start of each sector on each track. This is called low-level formatting, and sometimes is called "true formatting" since it is actually recording the format that will be used to store information on the disk.

The second formatting step is high-level formatting. This is the process of creating the disk's logical structures such as the file allocation table and root directory. The high-level format uses the structures created by the low-level format to prepare the disk to hold files using the chosen file system.

For a hard disk, there is an intermediate task that is performed between the two formatting steps: partitioning. For this reason, combined with the incredible complexity of modern hard disks, they are low-level formatted by the manufacturer, and high-level formatting is done by the DOS FORMAT command (or equivalent). Floppy disks require no intermediate step, and due to their relative simplicity, they are both low-level and high-level formatted at the same time by default when you use the FORMAT command.

Once the floppy disk has been low-level formatted, the locations of the tracks on the disk are fixed in place. Since floppies use a stepper motor to drive the head actuator, the floppy drive must be aligned properly in order to read the tracks on the disk. Sometimes the heads of a particular drive can become out of alignment relative to where they should be; when this happens you may notice that a disk formatted on the misaligned drive will work in that drive but not in others, and vice-versa.

Since floppy disks tend to be put together cheaply these days and many of them are getting rather old, it is generally preferable to always low-level format a disk in the drive you plan to use to write to it. If you have a disk that was formatted in another drive that is not working in yours, you can sometimes make it work again by reformatting it. (I personally never bother due to the cost of floppy disks; if I see errors it is gone.)

Tip: Many companies today sell preformatted floppy disks for basically the same cost as unformatted ones. These can save you a great deal of time if you use floppies a lot, and I have rarely encountered problems with them. If you routinely have trouble with good-quality preformatted disks your drive may be out of alignment or need cleaning. Make sure you buy PC formatted disks and not Macintosh ones!

Next: Floppy Disk Geometry

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