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[h1]You need to have the game(s) you are going to play from an External HDD already backed up but to do this let's make the correct file structure in our External HDD first.[/h1]

Go to [b]Steam program ==> Settings ==> Downloads[/b]
Click on the 'Steam Library Folders', it will open a new window, now select 'Add Library Folder' and browse for your External device and create a new folder in there then select ok.
go to the newly created folder and create another folder named "common" in there.