From MSoft :
When you run the Chkdsk utility program in read-only mode on a volume that uses the NTFS file system,
Chkdsk may not detect corruption in the disk structure.
This behavior can occur if the $logfile data region in the Master File Table is damaged.
To work around this behavior, run Chkdsk with the /f option to reinitialize the $logfile data region
and to correct the corruption problem.