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Article from Issue 78/2007

Although a couple of scratches are visible, you can read from the backup CD. The question is, for how long? Cdck measures the sector access times to give you an idea. Have you overwritten the bootloader on your disk? Super Grub Disk to the rescue!

CDs and DVDs use optical structures of approximately 100 nanometers to store data. Even tiny scratches can destroy large numbers of data bits, without the user even noticing at first. Data redundancy and error-correcting techniques, such as cross-interleaved Reed-Solomon code, support error-free data reading, despite damage to parts of the disk. Even newly burned media might not be free of errors: depending on how well (or how badly) your burning device supports your choice of media, the disks could have unreadable sectors right from the outset.

It would be nice to know how much of the existing redundancy has already been used up, or how grave the danger of imminent data loss is. The test functions that many burning tools use for media verification are not a big help. The functions only check whether the filesystem is legible after applying error-correction techniques.

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