2.7.3 2m

The 2m format was originally invented by Ciriaco Garcia de Celis. It also uses bigger sectors than usual in order to fit more data on the disk. However, it uses the standard format (18 sectors of 512 bytes each) on the first cylinder, in order to make these disks easier to handle by DOS. Indeed this method allows to have a standard sized boot sector, which contains a description of how the rest of the disk should be read.

However, the drawback of this is that the first cylinder can hold less data than the others. Unfortunately, DOS can only handle disks where each track contains the same amount of data. Thus 2m hides the fact that the first track contains less data by using a shadow FAT. (Usually, DOS stores the FAT in two identical copies, for additional safety. XDF stores only one copy, but tells DOS that it stores two. Thus the space that would be taken up by the second FAT copy is saved.) This also means that you should never use a 2m disk to store anything else than a DOS file system.

Mtools supports these formats only on Linux.