You do not always have an access to the files, even you are an administrator. In Windows, there are permissions, but also ownership. For example, on Windows Vista and later, if you try to remove a system file as an administrator, you will not be able to do it, since you are not an owner of this file, and the owner (one of the systems accounts, don't remember which one) did not grant you a permission to modify or remove the file.
Now, gladly as an administrator, you can change the owner of a file. Go to:
File Properties → Security → Advanced → Owner → Edit.
Select your name in "Change owner to" list and click Ok on every opened window (confirming all those annoying message boxes which will appear). Now you can grant permissions you want to yourself and access, remove or modify the file.
What do you mean when you say "and Security Tab is already missing"? Do you have Security tab for other drives? I'm pretty sure this tab is here even when a user does not have an access to the file, directory or a whole disk, at least for an administrator.
Probably it is something to do with your options. Maybe "Simple file sharing", a crappy thing like this which made everything really difficult when enabled under Windows XP?
Best Answer
Already mentioned in the comments, FAT does not offer any Security in terms of permissions. Change your Filesystem to NTFS and you will see the security tab. Before you change the Filesystem, save all your Data somewhere else.