BE is running as a service so it cannot see mapped drive.  To backup to a shared folder use the UNC name, i.e. \\servername\foldername and not the mapped drive.

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I tried this and then BE fails to backup with misterious errors,
i can connect to the drive when i add the folder as a media, but during the night it say's the drive is not there. so i have checked the password and the username for the NAS, everithing looks ok.
I read something about trusted device but i don't understand what this means

Why don't you test a small backup to that drive during the day and observe what is happening?  Also, post the BE error messages.