How can I get the permissions to write to the external SD card ...?

[zetabeta]

both ubuntu and pinephone use sudo system. probably you need sudo on both devices.

Code:

# user password is given
sudo chmod {parameters}
sudo chown {parameters}

in realty users and groups are store as numbers in a filesystem, then operating system determines which number is for which user and group.

Code:

# this gives root console, user password is given
sudo su -l
# or
sudo -i

# gives root, root password is given. (does not work on most pinephone systems).
su -l