The issue behind failing to mount ISO files is, it has got a sparse flag set. You can read more about this particular file attribute at this MSDN page( -us/library/windows/desktop/aa365564%28v=vs.85%29.aspx). In a nutshell, this sparse flag is facility supported in NTFS file system that enables efficient use of disk space by not writing zeros in a data stream. Instead it maintains an internal list containing the location zeros in file.