The root directory of my newly mounted drive appears to lack basic permissions, I can’t even move files there using Nautilus for instance. This was caused by accidentally transferring all data from a protected Clonezilla directory there.
I have often needed to do that, and a simple
cd /mnt
sudo chown user:user primary
works to give the user access to the root of that mounted device. What permissions you have beyond that depend on the device itself.