// computing values for extra sector
byte[] sectors = new byte[ 10, SECTOR_SIZE ];
// ... code to fill in values of sectors
byte[] extra = new byte[ SECTOR_SIZE ];
for ( int i=0; i<SECTOR_SIZE; i++ )
   {
   extra[i] = 0;
   for ( int j=0; j<10; j++ )
      {
      extra[i] ^= sectors[ j, i ];
      }
   }