Our renders run in a posix environment which is incompatible with Windows’ drive letter based file pathing. We do support UNC paths, should that be how your environment. is configured. We hope to provide support for drive letters in the very near future, we apologize for any inconvenience. In the meantime, you can certainly use file paths like:
to reference files on your C drive. Note that you would have to open the Maya or Nuke scene from a path written like this as well.