Bad sector

A bad sector is the result of mechanical damage. Bad sectors are a threat to information security in the sense of data remanence. Very often physical damages can interfere with parts of many different files.

In the 1980s, many software vendors mass-produced floppy disks for distribution to users of home computers that had bad sectors deliberately introduced. The disk drives for these computers would not read the sector: the header information may be duplicated so that different data was read at each pass from different physical sectors with the same headers, or the data in the sector would not be read correctly by the head, and various other techniques described above. The home computer equipment could only write "good" sectors, so that attempts to copy the disk were flawed either because:

• A sector was deliberately made "bad" so that the disk controller would attempt to read it several times, generally requiring one complete revolution of the media ("spin") for each attempt. This made reading slow, and the read would complete eventually indicating an error, were the disk legitimate. Were it a copy, it would complete quickly indicating a successful read: but this then proved it was a copy, made without the deliberate bad sector.
• The same header information was present on the same track more than once for the sector, typically half a spin (180°) apart, depending on the slew rate of the disk and the expected interleaving by the operating system. (Typically, disks are laid out so that the "next" sector to read will be about to pass the head just as the software asks for it.) So the head would read the "same" sector with different information, since two copies were available diametrically opposite and the disk head would see either of the two, depending on when it was asked. Generally, because of variations in spin speed, the request was made three or four times until different results were (or were not) achieved. Again, if the same data were retrieved every time, the disk was a copy; if different data were obtained, it was an original. In both cases the data was successfully read, so a simple XOR of the two (or similar) could then be used to give a comparison against a known string of characters, so that not only did the data have to differ, but had to differ in an exact bit pattern.

These techniques could generally be easily circumvented since the code to read the bad sectors was usually in the bootstrap loader on the disk itself, so by reverse engineering and rewriting the bootstrap loader, it would not look for the bad sectors, and the comparison for a known bit pattern would have to be encoded there, too.

• Sector slipping – a method of avoiding bad sectors
• ARccOS Protection – bad sector protection on DVDs, used by Sony.

• Bad Blocks Definition