If you go to your file manager in MAC, you should be able to open it to the Nucleus server and under there will be a storage tab, and under the storage tab will be the USB drive. Once there, just copy music to it.
Try using “guest” (without the quotes) as both the username and the password in Finder to access the Data directory - the external storage should be a subfolder there; e.g. Data\Storage\External SSD