It is not a good idea to change ownership and permissions on files from another operating system on a SD Card that is attached with a card reader to your computer. At least it affects security because a user has access to maybe password containing files and other important things. You also changed drastically permissions by removing all of them for
others. This can break the operating system because it is possible that it missed permissions to access or execute files managed by group members or allowed for general access. You should revert all ownership and permissions to its original state and never touch it when managing the SD Card on your computer.
Because you are able to change ownership and permissions on your computer, I assume you are running a Unix like operating system on it. To manage the files on the SD Card just do it with root rights. You can prepend commands with
computer ~$ sudo cp /source/path/new_wpa_supplicant.conf /media/myuser/rootfs/etc/wpa_supplicant/wpa_supplicant.conf
You can also switch to a root shell with
sudo -Es. Then you don't need to prepend
sudo. But have attention with it because you are root now. Any serious command will also be executed. If you do not have
sudo available you can just execute
su to become root.