40-track mode

Lore & Background

A 3.5 inch 1.44MB mini-floppy diskette contains 80 tracks, 18 sectors per track, and 512 bytes per sector. A 3.5 inch 720k diskette contains 80 tracks, 9 sectors per track, and 512 bytes per sector. This technique refers to formatting an 80-track 1.44MB diskette as a special 40-track 720KB diskette. In doing so one may create, in effect, two 40-track partitions. The former of these partitions is visible and usable as in a normal diskette, and the latter hidden. One may then fill the unallocated (hidden) 40 tracks with up to 720KB of secret or encrypted data, that will not be superficially visible to a user. Writing a 1.44MB floppy in 40-track mode causes the allocated tracks to be written to the actual even numbered tracks, thus causing drives attempting to read a 1.44MB diskette as a 720KB diskette to become confused because of the strange data in between even numbered tracks. The hidden data then resides on the odd numbered tracks. Generally, device drivers only copy allocated data, and thus traditional copies of such a disk would generally not contain the hidden data. Equivalents of this technique can easily be done on almost any media, for example, it is possible to hide audio in the pregap of the first track of a compact disc, or to hide data in the host protected area of a hard drive. This technique is different than the '40th track' copy protection schemes used during the 80s and 90s.

Reader's Guide

40-track mode is notable as a steganographic method that exploits the formatting differences between 1.44MB and 720KB floppy diskettes. By formatting a high-density diskette as a low-density one, it creates a hidden partition on the odd-numbered tracks that is invisible to standard operating system operations and traditional copying. This technique was used by the KGB and senior FBI agent Robert Hanssen to communicate with one another between 1985 and 2001, demonstrating its practical application in espionage. Its significance lies in its simplicity and effectiveness: it requires no special hardware, only a standard floppy drive and the ability to format a diskette in a non-standard way. The technique also highlights a broader principle of steganography—hiding data in areas that are not normally accessed or copied. Equivalents exist on other media, such as hiding audio in the pregap of a compact disc or data in the host protected area of a hard drive, showing the concept's adaptability. Unlike the '40th track' copy protection schemes of the 80s and 90s, 40-track mode is purely a concealment method rather than a copy prevention mechanism.

Did You Know?

• 40-track mode creates two 40-track partitions on a single 80-track diskette.
• Equivalents of this technique can be done on compact discs and hard drives.