I have a machine (installed ~2-3 years ago) with three user accounts. First user (id: 1000) is able to mount an USB stick via nemo (within budgie), but another user (id: 1001) gets "permission denied". To me there is no difference (except their user name/id, of course) between them, their group memberships for example are equal.
I tried another machine (installed within ~6months) and there all users can access USB media through nemo.
Any hints where to look next?