Sounds like you found the answer.
/home/public/data
/home/user_1/data
/home/user_2/data
...
extra (2nd or more hard drives) or external drives (NFS, CIFS, samba) are at:
/mnt/device/NN/public/data
/mnt/device/NN/user_1/data
...
where NN is the device id. These are auto mount devices so the device id may not show up initially. If you list one of the data sub-directories you can see the device IDs.
Example:
linuxmce@dcerouter:~$ ls /mnt/device
36
linuxmce@dcerouter:~$ ls /home/public/data/videos/
Internal HDD-CORE (sdb1) [34] NFS Media 1 [36] tv_shows_0 tv_shows_1 tv_shows_42
linuxmce@dcerouter:~$ ls /mnt/device/34
lost+found public user_1 user_2
linuxmce@dcerouter:~$ ls /mnt/device
34 36
Ownership should be root:public
linuxmce@dcerouter:~$ ls -l /home/public/data/videos/Internal\ HDD-CORE\ \(sdb1\)\ \[34\]/Science\ Fiction/The\ Matrix.*
-rwxrwxr-x 1 root public 8393392128 2008-01-08 19:33 /home/public/data/videos/Internal HDD-CORE (sdb1) [34]/Science Fiction/The Matrix.dvd
-rw-r--r-- 1 root public 18432 2008-02-19 02:41 /home/public/data/videos/Internal HDD-CORE (sdb1) [34]/Science Fiction/The Matrix.dvd.id3
That's why you need to sudo to copy the media.
Have fun,
Roy