file external change definition

Hello, could you tell me how phpstorm defines that file has been change externally ?

Here is my situation, recently I configured virtual machine with freebsd via virtualbox, all the time I've been developing on windows, so to save backward compatibility I mount my code folder (from windows machine) via smb to freebsd and use it in apache. Everything is fine but when I try just to read file in freebsd via apache or just f3 in midnight commander I got "external change" in phpstorm. And when I try to press ctrl-z I'm getting this message

I checked up - no attributes or file times which I can get via standart windows explorer properties aren't changing, maybe could you clarify the algorythm which defined that the file is changed.
Thank you in advance

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