Where is the downloaded Ardor blockchain stored on windows and linux? Where can one find the location of the Ardor blockchain on Windows and Linux?
On Windows (when using Ardor mainnet) the blockchain will be downloaded to your "C:\Users\USERNAME\AppData\Roaming\Ardor\nxt_db" folder. Make sure that you can see hidden folders. You can find your AppData folder very quickly by entering "%appdata%" in the address bar of the file explorer. 

On MacOS (when using Ardor mainnet) your default user directory is ~/.ardor so the blockchain should be in ~/.ardor/nxt_db

For Linux, you usually use the Platform Independent Zip Package and the blockchain is located in the nxt_db directory in the directory of the unzipped package