I am using a laptop.
The disk has two partitions C and D for Windows and some others where I installed Ubuntu 12.04. Windows partition D is called "DATOS". It is the place where my wife's data directories are stored.

Besides my Ubuntu user, who is an Administrator, I added a Standard user for my wife, called "Diana".
All works fine, but Diana user can not access partition DATOS.
I can access DATOS via my Home Folder without mounting it first, Ubuntu takes care of that chore.

How can I get Diana access to DATOS without changing her user type?

If you can access the partition data as root it is likely just a permissions issue as you state that Ubuntu is auto mounting it. If the partition is displayed in the root file tree you should be able to change the permissions in the properties dialog of the partition drop down menu to allow her access.

I did that, but machine does not like it.
Right-clicked DATOS / Properties / Permissions / Others Folder access:Create and delete files

Machine returns to "None"

I tried to change "Group", machine says:
The group could not be changed.
You do not have the permissions necessary to change the group of "DATOS".

That would indicate that you are not performing the action as root user as suggested in the post above.
What is the filesystem type on that partition? Open a terminal and run: df -T and it will show if the partition is mounted.

Here is the output for df

eduardo@eduardo-Intel-powered-classmate-PC:~$ df -T
Filesystem Type 1K-blocks Used Available Use% Mounted on
/dev/sda7 ext4 49214272 42307140 4407140 91% /
udev devtmpfs 500320 4 500316 1% /dev
tmpfs tmpfs 203304 1004 202300 1% /run
none tmpfs 5120 0 5120 0% /run/lock
none tmpfs 508256 352 507904 1% /run/shm
/dev/sdb1 vfat 7586604 7572532 14072 100% /media/PENDRIVE
/dev/sda6 vfat 52828000 51760176 1067824 98% /media/DATOS

You might read the link below:

http://askubuntu.com/questions/11819...-a-fat32-drive

Did you notice that partition is 98% full so you are heading for problems unless you start deleting some files.

Thank you, @yancek.
I've fixed the full partition problem by means of bleachBit and by emptying recycle bin.
After reading your link I concluded that the cost in effort and risk involved for a newbie like me for sharing a FAT partition don't match the benefit. So I will mark this thread as "Solved".
Thanks, again.