How Can Vim and Windows See Two Different Files that Have the Same Name in the Program Files Directory

I'm seeing a strange interaction between vim, the Cygwin tools, and Windows7. I don't think that it is a bug, but at the same time, I have no idea how to explain what I'm seeing.

I installed Vim (from vim.org, not the Cygwin vim) in:

"C:\Program Files (x86)\Vim".

I cd'd to this directory and edited my _vimrc file using vim itself.

vim _vimrc

I added some settings, wrote the file out, and exited.

I then copied the file to a different directory to create a backup of my changes.

copy _vimrc c:\tmp

That's when I noticed the issue. The copy of the file did not contain my changes. Much investigation followed. I will summarize the findings.

Within "C:\Program Files (x86)\Vim", I see my changes if I inspect the file with any of the following:

vim, cat, less

(Here, cat and less are the Cygwin versions.)

However, I do not see my changes If I inspect the file with any of the following:

notepad, type, more

(The commands type and more are standard Windows shell commands.)

To give you a flavor:

c:\Program Files (x86)\Vim>ls -l _vimrc
-rwx------+ 1 carlx Domain Users 936 Dec 23 21:15 _vimrc

c:\Program Files (x86)\Vim>dir _vimrc
 Volume in drive C is OSDisk
 Volume Serial Number is 6C86-85EB

 Directory of c:\Program Files (x86)\Vim

06/28/2011  02:09 PM               901 _vimrc
               1 File(s)            901 bytes
               0 Dir(s)  95,964,721,152 bytes free

The Cygwin ls command shows a different file (different date and size) than the Windows dir command.

I thought that perhaps it was some sort of issue with the casing of the filename, but specifying _VIMRC versus _vimrc to any of these commands, makes no difference.

Can anyone explain what I'm seeing here?

As Raymond Chen and @kreemoweet pointed out above, this is caused by filesystem virtualization. To turn this off, you can use the Local Group Policy editor, gpedit.msc. Type start gpedit.msc into a command prompt and navigate to Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options. Scroll to the bottom of the right-hand window and double-click on User Account Control: Virtualize file and registry write failures to per-user locations. Change the setting from Enabled to Disabled.