sector CRC

From: Mellen, Christopher (cmellenat_private)
Date: Thu Aug 23 2001 - 17:21:25 PDT

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     If I understand it correctly the CRC at the end of the data area of a
    sector is generated when a sector is written to.  Subsequently, the CRC is
    checked when the sector is read - if the CRC value doesn't match then it's
    flagged as bad and some type of error should occur.
    
    So my question, if I use a disk utility such as diskedit and change the data
    area (i.e. add a line of text to a document) in a sector.  Why then does the
    sector CRC not report an error when accessing the file normally at a later
    time?  or is this done with an ECC and it's correcting it on the fly as
    there would be no physical issues with the sector...?
    
    thanks
    
    
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