Change the default Downloads directory

Nautilus-the-file-browser is smart enough to notice that you're renaming a "special" folder ("Downloads" in your case) and adjust its settings to use the new name.

The settings are stored in the file $HOME/.config/user-dirs.dirs - you can either edit this or just do some trickery with renaming your current Downloads2 back to Downloads via Nautilus.

Then you can move the contents from there to a separate partition and then mounting that partition as $HOME/Downloads.

See this question for more details.

Use xdg-user-dirs-update

xdg-user-dirs-update --set DOWNLOAD /absolute/path/to/new/download/folder

Ubuntu Tweak can be useful for you this time, but includes the install of the Ubuntu Tweak software, whose intuitive GUI will guide you on this and some other useful processes.

When installed, simply choose Ubuntu Tweak from within the System Tools sub-menu in the main menu. After which you can go to the "Personal" section in the sidebar and look inside "Default folders", where you can choose which will be your default folder for Downloads, Documents, Desktop, etc.

I hope this help you.

Good luck!