My application tries to backup a NTFS volume and it tries to backup only used clusters.
I am using FSCTL_GET_VOLUME_BITMAP to get the used clusters.

I see that this call does not report 1024 clusters (4MB) and hence when I restore backed up image it is corrupt.

After I run chkdsk on this volume my restored files comes back.

For a 2GB volume FSCTL_GET_VOLUME_BITMAP returns 8716 clusters (of 4k each)
which translates to 35700736 but the windows volume properties show 39895040 as used space. the difference is 4MB or 1024 clusters.

Can anybody tell me what extra clusters I need to copy?