? did a back up

WD External Drives External Drives for PC
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MY BOOK ESSENTIAL DID A BACK UP THEY ARE SHOWING ALL BLUE WHICH I UNDERSTAND THAT IT HAS COPED MY FILES TO THE EXTERNAL DRIVE G . NOW MY C DRIVE FROM MY DESK TOP COMPUTER IS SHOWING 6 MB FREE SPACE HOW OR CAN I DELETE SOME FILES FROM MY C DRIVE TO FREE UP SOME SPACE? & HOW DO I DO THAT , THANK YOU