monline

Since this is invoked by one of the startup scripts of your OS, the exact behaviour might be a bit different depending on the implementation of your OS (and the specific OS version, eg. Ubuntu had this feature in /etc/init.d/checkfs.sh for quite a long time, then moved it somewhere else). The point is that at some step of your boot process the fsck tool is started with a set of commandline parameters. Eg. Ubuntu Hardy executes something like this:

Having a bootable DOS partition on an USB drive can be necessary for flashing the BIOS of your motherboard or RAID controller (since most recent PCs come without a floppy drive). However the official way to do this requires you to have already a DOS bootable partition. Fortunately there're a couple of solutions in case you've only a running linux system. The linked page has the best description of the qemu+FreeDOS method (the one that I prefer) and lists various other approaches as well. To keep the instructions safe, I made a copy of them here.

Many utilities (eg. firmware patches for CD/DVD drives, RAID controllers, etc.) require you to boot into DOS. However a lot of configurations (especially servers) come with only an optical drive, so you cannot use a floppy to boot into DOS. Not even to talk about the capacity of a floppy disk ... a RAID controller firmware might not even fit onto a single 1.44MB floppy disk. Fortunately newer motherboards already support booting from an USB device, but putting DOS (or something equivalent) onto an USB drive and make it bootable is not a trivial task.