Security issue with owncloud, yunohost and data on usb drive

I have a small linux box with limited disk space so I placed the owncloud data on a usb drive.
Last Owncloud package (from yunohost ???) enables the users home directory as an external mount and my users can from here browse the usb disk and enhance access all the owncloud data directory with no restriction.
is there a way to tamper that. Besause using the home directory is not such a bad idea, just in my case it is not very good.

Yes, yunohost has the latest version of ownCloud (8.0.3); list of apps > OwnCloud > GitHub >

As for the mounting external storage thing, I don’t know, my yunohost instance is broken at the moment.

Is there a way to mount the disk so that only root and user owncloud can access it and so that it is not visible from users home?