Why some photo in Photos is linked to iPhoto only?!

Hi and tnxs for reading.

While testing some ways to manual backup my photos from Photos, I got a caution msg from Photos in relation to 2 out of 300 album I have.

This msg was stating that some photos cannot be exported since they are linked to iPhoto library and not Photos library. There are instructions suggesting me the way to link them to the appropriate folder (in this case folder n.10 of Master/2012 subfolder of iPhoto Library).

Of course this is something I do not like since I would like Photos to be independent from iPhoto.

Here are my doubts/questions:

1) why this happened?

2) by following the instruction to allow Photos to access those photos, I cannot proceed since when clicking "OK" I need to manually point a folder inside iPhoto library, which is greyed in the opened window.

3) Can I fix the issue by consolidating the 2 albums? By selecting the 2 albums and selecting "Consolidate" I get a warning saying that this action will copy original files in Photo library and the action is irreversible. Since I am not an expert on this, I am asking here before continuing...

Tnxs.