A bootable floppy is a diskette containing a back-up copy of your hard disk master boot record (MBR). In the event that the master boot record becomes "infected" by a boot virus, having a bootable floppy will allow you to load it back onto your hard disk. (Otherwise, you may have to reformat your hard disk which first erases everything on the disk including files you may not have a backup copy of. Even if you do, reformatting your hard disk will mean you have to reinstall everything you've backed up, a time-consuming procedure at the very least.)
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