Helping people with computers... one answer at a time.
In Windows Vista, and Windows 7, several common folders have been moved or renamed. The old names remain for compatibility (and confusion).
I use Vista which I find far more difficult to navigate than the XP used formerly. For some reason all my files seem to have duplicated themselves and I cannot access them. For example I have two folders entitled application data and another app data. I can access one but when I try to access the other I am told that the folder is inaccessible and access is denied. Did I perhaps inadvertently initiate a share or something equally stupid?
Nope, nothing you did at all, stupid or otherwise.
This is part of some kind of redesign that Microsoft initiated in Windows Vista that folks just now upgrading to Windows 7 are encountering as well.
And I agree, it's confusing. I can take some guesses as to why they both exist, but first let's look at exactly what they are, and perhaps more importantly, what they are not.
Application Data and AppData are almost the same thing.
AppData is the intended location for user-specific data that applications might want to keep. For example, if your machine has two users, "John" and "Mary", a program might save different settings or files depending on who's logged in and using the machine by placing those settings somewhere within the AppData folder for that user.
Within AppData, you'll find a couple of additional folders, most notably "Local" and "Roaming". "Roaming" folders are designed for networks that support what are called "Roaming Profiles" - a scenario where your documents and setting are available to you no matter what computer you log into in your corporate network, as long as you login with your unique username. "Local" are then settings that are only local to that specific machine.
It feels like overkill, since the majority of consumers
have only one login account on their machine
have never heard of, much less used, "Roaming Profiles"
They're both artifacts of Windows being designed as a multi-user operating system also targeted at large corporate installations.
"Application Data" is what you would find in Windows XP.
In Windows Vista and Windows 7, "Application Data" is actually an alias or a synonym (in geekier terms, a "hard link") to your "AppData/Roaming" folder.
You can actually prove this yourself. In a Windows Command prompt, you can write to the "Application Data" folder. This example simply echos the string "foo" into a text file "foo.txt" in the Application Data folder:
C:\Users\LeoN>echo foo >"Application Data\foo.txt"
And now, if we look at the AppData\Roaming folder:
C:\Users\LeoN>dir AppData\Roaming Volume in drive C has no label. Volume Serial Number is 50FF-9FCF
Directory of C:\Users\LeoN\AppData\Roaming 01/10/2010 10:17 AM <DIR> . 01/10/2010 10:17 AM <DIR> .. 01/10/2010 10:17 AM 6 foo.txt 08/26/2009 08:03 PM <DIR> Identities 11/02/2006 07:06 AM <DIR> Media Center Programs 08/27/2009 07:56 AM <DIR> Parallels 1 File(s) 6 bytes 5 Dir(s) 10,194,100,224 bytes free
You can see that even though we created the file in "Application Data" it appeared in "AppData\Roaming".
In fact, there are several aliases that were created as a result of some reorganization in Windows Vista: "Documents And Settings" became "Users", "My Documents" became "Documents", and others.
The bottom line is that even though the files might appear as if they're in two locations, they're not. The files are not duplicated, they're simply in a single folder that can be accessed via different names.
Now, the other question: why?
I can only speculate.
Why the reorganization? Probably to more cleanly allow things like Roaming Profiles to work, and support a cleaner division between information local to a machine or portable across multiple machines when Roaming Profiles are used. In the case of the average consumer there's no clear benefit or difference. (There may well be other reasons as well.)
Why different names? As I look at the new names, they share on common characteristic: they no longer have spaces in them. My belief is that they took advantage of the situation to remove spaces from within names, since it's common for people - and programs - to get confused by spaces in file and folder names. To the average consumer I consider this a pretty significant usability improvement, as there's no longer a concern about whether or not you need to worry about things like quotes around "Documents and Settings" when plain old Users will do.
Why the aliases? In a word, compatibility. There are programs that assume "Documents and Settings", rather than using the system provided functions to ask "where should I put user data?". Removing those names would probably break a number of applications.
Why the draconian permissions on those old names? To "encourage" (aka "force") users and future applications to move to the new/correct names.
So what it all comes down to is this: it's a reorganization that for the most part shouldn't really impact the average user.
Or to put it another, less useful way: it is what it is.