Using git for VS2010 project: Can't add file *.opensdf

I'm using git to track a C++ project in VS2010. I'm using ignore patterns found on stackoverflow, which usually do not exclude the *.opensdf file. Unfortunately, when trying to commit the file, I get the following error:

error: open("foo.opensdf"): Permission denied
error: unable to index file foo.opensdf
fatal: adding files failed

Why do I get this error and how can it be avoided? Since I'm quite a newbie with VS2010, what is the use of this file?

Thanks!

mort

*.opensdf is a temporary file opened only while .vcxproj/.sln is loaded to Visual Studio IDE. It should be added to your .gitignore file.

See the accepted answer on this question: What should be contained in a global source code control ignore pattern for Visual Studio 2010?