Your Arcade Content Storage location can be changed manually to a user defined directory. You can move the content to an external drive, or to a new location on your boot drive.
To do so, open the Settings page, click the Content Storage tab, and select Move To New Folder.
Select Browse, navigate to the desired location on your external drive, then click Apply.
You do not need to create a new folder in the new location, Arcade will create one for you called 'Arcade Content'. After copying your Arcade Content folder, you may delete your original content folder.
The original folder locations are as follows:
Mac: /Library/Application Support/Output/ARCADE/ARCADE Content
PC: C:\ProgramData\Output\Arcade\Arcade Content