What is the concentric circle on a floppy disk called?

Floppy disks have had an interesting history. They were developed in the late 1960s by IBM engineer Alan Shugart, who envisioned a portable and inexpensive way to load programs and store data. The first commercial floppy disk, called the “Type I Diskette,” was introduced by IBM in 1971. It was 8 inches in diameter and stored about 80 kilobytes of data. Over the next couple decades floppy disks shrank in size from 8 inches to 5.25 inches to 3.5 inches, becoming more compact and portable while also increasing in storage capacity. At their peak popularity in the 1980s and 90s, floppy disks were a ubiquitous storage medium for personal computers.

The floppy disk itself is composed of several components. At the center is a metal hub that connects to the read/write head in the disk drive. Surrounding the hub is an oxide coating that stores the magnetic data. The disk is encased in a square plastic protective shell. A metal shutter covers a cutout in the shell that allows the read/write head to access the disk. The shutter slides open when inserted into a disk drive. There are typically a series of holes at the corners of the plastic shell to indicate the type of disk and how much data it can store. An index hole near the hub allows the disk drive to detect the start position of each track.

Magnetic Media

Floppy disks store data on a thin magnetic coating that allows data to be written and rewritten. The disk is made of a flexible plastic coated on one or both sides with a ferric oxide material suspended in a polymer binder (according to https://web.mit.edu/2.972/www/reports/floppy_drive_read_write/floppy_drive_read_write.html).

This magnetic coating allows binary data to be stored by magnetizing tiny spots on the surface of the disk in one direction or the other. A read/write head inside the disk drive can detect the magnetization of each spot as it passes under the head, reading the 1s and 0s. The head can also flip the magnetization of a spot to rewrite the data (according to https://web.mit.edu/2.972/www/reports/floppy_drive_read_write/floppy_drive_read_write.html).

The flexible plastic disk coated with magnetic material enabled a new method of portable, reusable data storage and revolutionized computing in the 1970s and 1980s.

Disk Composition

The actual disk material inside the square plastic case is usually a thin circular plastic film or mylar. According to the Wikipedia article on floppy disks https://en.wikipedia.org/wiki/Floppy_disk, the 8-inch and 5 1/4-inch floppy disks contain “a magnetically coated round plastic medium with a large circular hole in the center for a drive’s spindle.”

The Made How article on floppy disks http://www.madehow.com/Volume-1/Floppy-Disk.html provides more details, explaining that a 3 1/2-inch floppy disk contains “several layers of liner and recording media sandwiched between two hard plastic cases.” The actual disk material that holds the magnetic information is a circular plastic film, usually made of mylar.

Metal Shutter

The floppy disk has a metal shutter that covers the opening that allows the read/write head to access the magnetic media inside the disk. When the disk is inserted into the disk drive, the shutter slides open to expose the magnetic media. When the disk is ejected, the shutter slides back closed to protect the magnetic surface from dust, debris, and damage when outside of the drive.

The metal shutter is composed of stainless steel and slides back and forth along tracks on the plastic housing of the floppy disk. Its purpose is to shield the magnetic media when not in use. When closed, the shutter prevents dust and contaminants from compromising the magnetic surface. When open, it gives the read/write head full access to magnetically record data onto the spinning magnetic disk inside.

Overall, the metal shutter is an important component that protects the floppy disk when not loaded in the drive. Its sliding mechanism allows it to open and close automatically upon disk insertion and ejection. This preserves the integrity of the magnetic media surface and prevents deterioration of the recorded data.

Central Metal Hub

Located at the center of the floppy disk is a circular metal hub that allows the disk to spin within the floppy drive. This metal piece has a hole in the center that fits over the spindle of the drive motor so that when the motor spins, it rotates the disk. The hub is typically made of steel and is attached to the underside of the floppy disk’s fabric-reinforced inner circle. The hub transfers the rotational force of the motor to the disk so that the read/write heads can access different tracks on the magnetic media.

Without the central metal hub, the floppy disk would have no way to spin at a constant rate within the drive. Older 8-inch and 5.25-inch floppy disks had larger hubs made of aluminum, while 3.5-inch disks use smaller steel hubs. The hub diameter matches the spindle size of the corresponding floppy drive model. For example, 5.25-inch drives use 17.5mm spindles and hubs, while 3.5-inch drives use 12mm spindles and hubs. The hub-spindle interface is one of the factors ensuring compatibility between disks and floppy drives.

Source: https://computer.howstuffworks.com/floppy-disk-drive2.htm

Track-Indicating Holes

Floppy disks have rectangular holes in the rigid casing that allow the drive heads to detect tracks. These track-indicating holes line up with the concentric magnetic tracks on the flexible disk inside the casing. As the disk spins, the holes pass under the drive heads and allow light to shine through, which is detected by sensors in the drive. This provides feedback to the controller about the current track position of the heads.

Typically, floppy disks have two track-indicating holes, one for each head. For double-sided disks, the holes are on opposite sides of the casing so that both heads can detect tracks independently. The location of the holes corresponds to specific track numbers, enabling the controller to identify each track position accurately.

Write Protect Notch

The 3.5-inch floppy disk had a small rectangular notch on the side of the rigid casing. This notch was paired with a sliding plastic tab that could cover or expose the notch. When the notch was covered by the sliding tab, it would prevent the floppy drive from writing new data to the magnetic disk inside the casing. This write protection mechanism allowed users to protect the contents of a floppy disk from being overwritten accidentally.

The write protect notch and sliding tab were physical means of controlling access to the data, similar to how a read-only file can prevent modifying file contents in software. When the plastic tab was slid to cover the notch, it would mechanically block the floppy drive’s write head from touching the media. The drive could still read existing data on the disk, but was prevented from writing and modifying it. Many operating systems like Windows would also show a disk as read-only in the UI when write protected.

Users often enabled write protection when finished copying important files to a disk, to avoid accidentally deleting or overwriting them later. The position of the sliding tab made it easy to check and toggle the write protection setting.

Concentric Magnetic Tracks

Floppy disks contain concentric magnetic tracks where data is stored. These tracks are arranged in concentric circles across the surface of the disk, similar to the rings on a tree (Wikipedia, 2022). The tracks are separated by gaps so that the read/write head can easily move between them.

Each track is divided into sectors that store a fixed amount of data. More tracks on a disk allow for greater storage capacity, as there is more area to save data. For example, an 8-inch floppy disk could hold up to 80 tracks, while a 5.25-inch disk typically had between 35-70 tracks (Computer History Museum, 2022).

The number of tracks and sectors per track increased over time, allowing newer floppy disks to store more data in the same physical space. For instance, double-sided and high-density floppies were introduced to pack magnetic tracks more densely for expanded capacity (HowStuffWorks, 2022).

Answer: Index Hole

The circular hole located near the center of a floppy disk is called the index hole. This hole allows the floppy disk drive to control the speed of the disk as it spins. As the disk rotates, the drive uses a sensor to detect the passing of the index hole. This provides a reference point that enables the drive motor to spin the disk at a constant velocity (Krebs).

The index hole is essential for the proper functioning of a floppy disk. As the read/write head moves across the surface of the disk to access data, the speed must remain steady. Fluctuations in velocity would cause errors and data loss. So the index hole, in conjunction with the drive motor and a feedback circuit, ensures that the angular velocity stays within acceptable parameters (McComb).

The size and location of the index hole differs depending on the type of floppy disk:

  • 5.25″ floppy disks – The index hole is at the upper left of the central hub.
  • 3.5″ floppy disks – The index hole is a small circle between the hub and the upper edge of the magnetic media.
  • 8″ floppy disks – The index hole is a rectangular notch on the right side of the central hub (Retro Computing).

So in summary, the index hole is the essential concentric circle on a floppy disk that allows the drive to control the disk’s speed during read/write operations.


Krebs, Brian. “What Floppy Disks and Typewriters Tell Us About Cyber Resilience.” Technology review, 18 Mar. 2022, www.technologyreview.com/2022/03/18/1047019/legacy-technology-floppies- keyboards- cybersecurity/.

McComb, Gordon. “Tech Explained – the Floppy Disk.” Tech Explained – the Floppy Disk, www.explainingcomputers.com/floppy.html.

Retro Computing. “Floppy Disk / Diskettes.” Floppy Disk / Diskettes // retrocmp / retro computing, retrocnp.de/fdd/diskette/diskette.htm.

Summary of key parts of a floppy disk

The key parts of a floppy disk that allow it to store data are:

  • Metal shutter – protects the disk when not in use
  • Central metal hub – allows the disk to spin
  • Track-indicating holes – allows the drive to detect the start of each track
  • Write protect notch – prevents accidental writes to the disk
  • Concentric magnetic tracks – stores the actual data

Importance of index hole in reading data

The most critical part is the index hole. This is a concentric circle near the metal hub that allows the drive to detect the start of the first track. Without this index hole, the drive would not know where to start reading the data from the magnetic tracks. It acts as a reference point so the drive can properly locate the data on the rotating disk.