Not showing(HIDED) old backup folder in media disk

hi..i am using backup exec 11d symantec for backupS,before two day i had taken backup in media which having more than 450 GB  FREE space disk,and on that media already HAS A backup data  near 525 GB data,

but when i taken 155 GB backup  on  that  media  afterthat  oldbackup data are hided...HOW CAN SOLVE THIS PROBLEM...