Hi,
we will not develop the material library further (except for adding new materials), and instead it will be folded into Chaos Cosmos in future versions.
You can now simply move the library anywhere, e.g. D:\, and use mklink /J "C:\Program Files\Corona\materials" D:\materials command on windows to create a junction ("redirect"). Everything will work as if the folder was in C:/, but it will be stored in D: