SOLVED : Java - filepath - Invalid escape sequence

It's a compile-time error, so it can't be to do with permissions etc.

The problem is that you're not escaping the backslashes. You need:

String filePath = request.getRealPath("\\\\10.0.1.18\\downloads\\upload)";

Then the contents of the string will be just

\\10.0.1.18\downloads\upload

This is exactly the same as in the first line you showed, where this:

String filePath = "D:\\location";

... will actually create a string with contents of:

D:\location