Board Thread:Questions and Answers/@comment-82.30.149.148-20121218174721/@comment-5112555-20130901083324

yup, if minecraft is open, it is using the folder, and when a program (minecraft) uses a folder, another program (the program you use to delete) can't make changes in it, because if the first progam (minecraft) is using a file that suddenly gets changed, it will have to shut down. and becuase your OS wants to prevent that from happening. (it can't see the difference between minecraft and for instance your antivirus program, so it just makes sure you don't really screw up your PC)