From Aperture (external HD) to Photos

Now i realized that with Photos all the photos are going to be saved into iCloud.

They are not unless you choose them to be. It's an option. You don't have to use it.

My question is: can i use Photos from an external HD where my aperture library is saved?

Yes.

You can also have multiple Photos 'libraries' dotted around your local storage as you could with iPhoto/Aperture - only one library will sync with the cloud, and then only if you enable that option, it does not appear to be mandatory.

Can anyone tell how photos are stored in the new Photos?

I moved from iPhoto to Aperture because I wanted to organise my original photos in my own way on external HDs.

With iPhoto the program added them in a (to me) strange folder system and I could not figure out how to find my photos from outside the application.

Two questions:

What will happen if I go from Aperture to the new Photos. Will I still be able to decide where to store my originals?

And will the new Photos, like the old iPhotos, create a duplicate every time I make an adjustment to a photo?

Anyone?

Will I still be able to decide where to store my originals?

Photos can manage referenced original image files the same way as iPhoto did. When you import files, you can opt to leave them in their current location and reference them there. But apparently Photos has no way (yet) to relocate or consolidate referenced files. I would not risk a referenced library with photos currently.