The IBM eXtended Density Format (XDF) is a way of superformatting standard high-density 3½-inch and 5¼-inch floppy disks to larger-than-standard capacities.
It is supported natively by IBM's PC DOS versions 7 and 2000 and by OS/2 Warp 3 onward, using the XDF and XDFCOPY commands (directly in OS/2).
However, the first cylinder uses standard formatting, providing a small FAT12 section that can be accessed without XDF support and on which can be put a ReadMe file or the XDF drivers.
Floppy distributions of OS/2 3.0, PC DOS 7 and onward used XDF formatting for most of the media set.
Thus, USB attached floppy drives cannot read XDF formatted media.