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The awnser: diskpart.exe!

1. Hit “Windows Key” + “R” to open the run dialogue box and type “diskpart” and hit “OK” to open a command prompt window.
2. Type “list disk”.
3. Type “select disk n”.
4. Type “list part”.
5. Type “select part n”.
6. Type “delete partition override”.
7. Type “exit”.

Thanks to http://www.disk-partition.com/blog/how-to-delete-an-oem-partition/