Fourth Pass: Sectors
The fourth pass of Chkdsk occurs only if the /r command-line switch is used. If the /r option is used, Chkdsk runs a two-stage pass to look for bad sectors in the volume's free space. Chkdsk tries to read every sector on the volume to confirm that the sector is usable. Chkdsk always reads sectors that are associated with the critical metadata files to validate them, even without the /r option. Sectors that are associated with user data are read during earlier phases of Chkdsk if the /r command-line option is specified.
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Stage 1: Verifying file data. In this stage, Chkdsk tries to read all the user data. If a read on a cluster fails, Chkdsk allocates a fresh cluster in place of the bad cluster and adds the bad cluster to the list of bad clusters.
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Stage 2: Verifying free space. In this stage, Chkdsk sends a command to the controller to verify the free sectors on the disk. Chkdsk adds clusters that contain bad sectors to the bad cluster list.
If you use a software fault-tolerant disk, NTFS recovers data from the bad clusters and writes the data to the newly allocated cluster. If you are not using a software fault-tolerant disk, the new cluster is filled with a pattern of 0xFF bytes. Hardware fault-tolerant disk solutions are frequently able to remap the bad sectors, in which case NTFS does not have to also remap them.
If NTFS finds unreadable sectors during the course of typical operation, NTFS remaps the sectors in the same way that it does when Chkdsk runs. Therefore, using the /r command-line switch is typically not necessary. However, using the /r option is a convenient way to scan the whole volume if you suspect that a disk may have bad sectors.
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