I followed this guide to encrypt my home folder and now I cannot access my external hard drive. As this guide instructs, I created a temporary user with encrypted home and migrated all my files to that user. I also made sure that the new user belongs to the same groups as my old user. After some time I deleted the old user. Everything else aside from this mounting problem seems to be working fine.
Open Pantheon files as an administrator. Then change the owner of the
/media/magnus folder to the correct user. Now your user should have full access to mounted media.