Adding files to watched, from NAS - permission

When I'm mounting the octoprint watched and uploads folders on my Synology NAS, the only way I have managed to add it properly is with the username 'pi' and the password for my octoprint user (octoprint user name is not 'pi') But still, it seems like the folders are read-only, since I get a "you don't have permission .." when trying to add files to the folders. Does anyone know how I can solve this?