File Manager in Kodi is an important tool. Using this tool you may copy/move files around.
First step in using file manager is to apply below setting if not done already.
Enable "Show hidden files and folders" using the help from the below video.
Watch the below video, which explains the functioning of file manager in Kodi.
This video is for Kodi 16 Jarvis. But it applies for ver. 17.x Krypton also.
Open File Manager and if the internal and external HDDs are not visible, use Add Source command in File Manager to add these. Select one pane (either left or right) and navigate to the internal HDD and the folder containing media files. Select external HDD on the other pane. Now you may copy files from one pane to other.