The error correcting codes take approximately 5% of the available space on a floppy disk. They allow complete reconstruction of the data on disks that are only slightly damaged (say 4 or less sectors are unreadable, or one complete track is unreadable), and often allow partial reconstruction when there are much more unreadable sectors.
Of course there are limits to what error correcting codes can do. If 6% of the data on a floppy is lost, there is no way to reconstruct it all. But with proper floppy disk care, the chance of this happening is very small, much smaller than the chance that a few unreadable sectors will develop. And even if 6% is unreadable, the tbackup compressed archive format will keep the effects localized.
Note that the availability of error correcting codes does not mean that tbackup can safely write to floppies that are known to have bad spots. Some backup programs know how to avoid bad spots when they detect them, but tbackup does not do this, mainly due to limitations in the Linux floppy drivers.
Tbackup will try to detect bad spots when writing a backup, and will reject a bad floppy if it finds one. Many kinds of bad floppies cannot be detected while writing, allowing some floppies that are known to be bad to be used for backups. Doing this is not recommended. Using bad floppies will always make an archive less safe, regardless of error correcting codes.
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