My only guess is that your script is running in a context where it doesn't have a current "working drive" and if you prepend the "C:" to your string, that it might fix the problem.

From that statement you didn't seem to be considering permissions as a possibility so I brought it up.

Thanks! I failed to update that assessment after I had read the code that I quoted. A permission problem did then cross my mind and it is one of the more likely causes so it is good for you to mention it.

Given that the OP has reported "I can access using command prompt", it seems an ordinary permission problem may not be the cause. But it could still be a permission problem if the script is being launched in a restricted context (such as as a service, from a web service, as a scheduled task, etc.).