I would recommend —before going wild with risky rm commands —that one closely examines a full listing of all trashed items first, to see precisely what is where... and which owner/group/flags/perms/ACLs the problem items actually have.
This command will measure trash size everywhere (except /var/root/.Trash):
sudo du -sch /Volumes/*/{,Users/*/}.Trash*
...and this one lists all the gory details:
sudo ls -GlObFReak /Volumes/*/{,Users/*/}.Trash*
[note: those commands assume we're running a Bash shell, or one capable of doing the brace expansions.]
If some file(s) supposedly can't be deleted... then at least let everyone see exactly what's going on.
[edit: hopefully the external volume isn't mounted as read-only.]
