If you try to drag and drop copy files from iCloud Drive’s Desktop or Documents to a non-Sierra Mac, you’ll see this warning.
To copy files from the iCloud Drive Desktop or Documents folders when using a non-Sierra Mac, you have to hold down the Option key while dragging and dropping, or right-click the file and select Copy, then on your non-Sierra Mac, right-click on the destination and select Paste Item.
Sierra is currently in beta, which means there’s a chance changes could be made to how this all works. This article will be updated appropriately.
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