nicholas@mordor:~/Desktop$ 
nicholas@mordor:~/Desktop$ id
uid=1000(nicholas) gid=1000(nicholas) groups=1000(nicholas),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),122(lpadmin),134(lxd),135(sambashare),137(docker),140(libvirt)
nicholas@mordor:~/Desktop$ 
nicholas@mordor:~/Desktop$ sudo mount -o remount,uid=1000 /mnt/external
nicholas@mordor:~/Desktop$ 
nicholas@mordor:~/Desktop$ cp foo.txt /mnt/external/
cp: cannot create regular file '/mnt/external/foo.txt': Permission denied
nicholas@mordor:~/Desktop$