The Windows partition is formatted in NTFS, and OS X can't write in it by default. If you want to transfer files to your Windows partition from OS X, you have to use a third-party application as Paragon NTFS.
However, when you install Boot Camp drivers in Windows, they include a HFS driver that allows you to read the OS X partition from Windows, so you can access to it and copy the files you want to the Windows partition
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