Measures for accident!?
 

What measures should I take to prevent from the accident while using linux? Someone tell me to make a boot disk. But I don't know how to use it. Moreover, I found the system can't boot from a bootdisk, why?

if the computer can't boot from a floppy, then that's probably a problem with the BIOS - you can just change them to try booting from the floppy before the hd.
the bootdisk should just have a small kernel image on it, enough to boot from, and might mount the rest of the drives... but it depends on the boot disk.