Timeout? while copying files

[Multicard]

We're trying to copy about 350K files to our RS411 (From windows 7).
But every 5-10 minutes we get a error "Too many files are currently in use. Quit one or more applications, and then try again.".
The files are not being used by other applications.

Current solution is to open a random folder on the RS411 and then it will open it after about 20-30 seconds. And then press try again and it will continue to copy. But this will happen again after 5-10 minutes. This also happens with less files.

Does anyone know why this happens and what the solution is?

Niels