Chkdsk activity has five phases. The first three are major passes during which Chkdsk examines all the metadata on the volume; the fourth pass, checking for bad sectors, is optional and has two phases.
During its first pass, Chkdsk examines each file record segment in the volume's MFT and displays the percent of verification that is complete.
A specific file record segment in the MFT uniquely identifies every file and folder on an NTFS volume. Chkdsk examines each file record segment for internal consistency and builds two bitmaps, one that represents the file record segments that are in use and another that represents the clusters on the volume that are in use.
At the end of this phase, Chkdsk has identified the space that is in use and the space that is available, both in the MFT and on the volume as a whole. NTFS keeps track of this information in bitmaps of its own, which are stored on the disk. Chkdsk compares its results with the bitmaps that NTFS keeps. If Chkdsk finds discrepancies, the discrepancies are noted in the Chkdsk output. For example, if Chkdsk finds that a file record segment that was in use is corrupted, it marks the disk clusters that were associated with that file record segment as “available” in the Chkdsk bitmap. However, because the same clusters are marked as “in use” in the NTFS bitmap, Chkdsk replaces the NTFS bitmap with the one that it generates.
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