Why did the rights change to 775 when transferring files?

Question

linux

I reinstalled the system and copied the necessary files to an external screw. After installing a fresh system, I copied them back using the system GUI (not from the console). Ubuntu 16.04.
Now I noticed that the file permissions have changed to 775
1. Why did this happen?
2. What is the risk?
3. How to fix? There are many files, all in different places.
4. How to transfer files correctly in the future, preserving the attributes?

Answer 1

On the external screw, I suspect, NTFS? When transferring between different file systems, the rights may be lost/changed.
When installing Linux, the easiest way is to make a separate partition for / home and then you won’t have to transfer anything, all user settings and files will remain untouched.