空間無喇喇冇左?(WINDOW 7)

我個C DRIVE 明明只用左20G 但我係我的電腦中看到只有20G FREE OUT OF 50G , 咁即係用左30G喇,咁個10G去左邊度?係唔係自動同我做左BACK UP呀?點解無喇喇冇左???