How does the security work?

This is probarbly a really dumb question but I’m pretty stupid when it comes to computers so bare with me. I just bought a My Book Live external hard drive and I don’t quite understand how the privacy on this thing works. I could easily access the drive from other devices without having to instal them or use passwords or anything. Is this because they connect through my router and the router acts as a security from outside users? I saw that the disk makes a difference between private and public shares. Is this a security level INSIDE or OUTSIDE my houshold? To sum up my main question here: is there any risk that people outside my household can access my stored files?

“I could easily access the drive from other devices without having to instal them or use passwords or anything.”

Only Public folder and only people connected to your own router/LAN

Is this because they connect through my router and the router acts as a security from outside users?"

Not clear question. People connected to you router are not “outside”, they are inside, aka LAN, same LAN.

People trying to connect via the internet, are “outside” and cannot connect by default, unless you allow them

“the disk makes a difference between private and public shares. Is this a security level INSIDE or OUTSIDE my houshold?”

only people within your LAN, has access. However, only Public folders have access from anyone. Private folders, you define the user access.

“is there any risk that people outside my household can access my stored files?”

Not by default, unless you allow it.

But stop referring to inside/outside, since it seems you ar confusing/mixing the two, at least until you grasp the whole concept terminology. Lets try this.

inside = connected to your router aka same LAN*

outside = from the internet aka WAN

*Why do I mention this? You could have people outside your home, connected to your router. Which means they are connected to the same router as you and are in the same LAN and have access to everything you do as well