62. The $BadClus

• Entry #8 in $MFT

• Tracks bad clusters in volume

• Cluster is bad if at least one sector is bad

• The $Bitmap marks bad clusters as 1-in use, preventing other files from trying to use the bad cluster in the future.

• The $BadClus has the size of the NTFS volume:

  • Organized as a sparse file of all zeros

  • Zeros in sparse files are counted instead of saved

  • $BadClus takes no space on the disk: if a cluster becomes bad, the data is written into $BadClus instead at the same offset as if it was written on disk.

• See also
  http://en.wikipedia.org/wiki/Sparse_file