chkdsk alone just looks for errors
chkdsk /f fixes errors
chkdsk /r will repair damaged sectors and attempt to recover the data on it - but if you have to use this, your HD is on the way out and you better start looking at buying a new HD because you old one isn't going to be around much longer.
-S
PS. It is a good idea to run a chkdsk /f once in a while. It will fix the occasional problem (though this is not supposed to happen on a journalising file system, but still) / data corruption that occurs over time. If weird things start happening to your computer, run this first since possibly a driver or application has become corrupted - this happens on NTFS too.
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