How to grant a user acces to one folder on external hard disk?

I plugged a disk into my FreedomBox and it is mounted automatically to /media/root. There I would like to create a folder, create a new user via “user and groups” app and grant the user read and write permissions but only for this folder and subfolders. Plan is, that the user can then access the FreedomBox SFTP Server locally and automatically get access to this folder. How can I do so?