Yes, by default the logged in user has r/w access to removable media.
1-)
Is it true for any filesystem ( ext4, fat, ntfs ) ?
2-)
I try to run a script as normal user ( with some command using sudo ) which need r/w access to a usb drive just formatted as ext4 filesystem.
I have no access to the usb drive which is mounted for user root:root.
To mount the usb drive, i just use the kde device notifier .
Any suggestions are welcome.
3-)
After running chown some_user:users, i have r/w access to the usb disk, even after reboot or log out.
I suppose if i log as another user, i will have to run chown for this new user ?
4-) If i run my script on another computer, I suppose i will have to run chown on the new computer for this usb drive ?
An ext4 file system (like any other Linux file system) has directories and files on it that have owners (by user and group) and permission its. That works exactly as it would work for any other directory/file on a Linux system. E.g. when a user is not the owner of the file and also is not attached to the group of the file, (s)he can only write to that file when the permission bit for w(rite) for o(thers) is set. Just very, very normal.
Non-Linux file systems do not have owners or permission bits. On mounting on a Linux system they are faked. What exectly is faked (which owner, whichj group, which permissions) is set using file system specific mount options. E.g. when a NTFS file system is mounted (with help of the desktop because it is hardware-wise connected during the session og a user), a mountpoint is created and the mount is done using the user “in the seat” and his default group and a set of permissions (often without the x bit) is used.