It could be that
a) the drive doesn't exist or isn't mapped
b) the control contains characters or words that are not allowed in folder/file names
c) the event you're using is not appropriate for this (you don't reveal what event it is)
If you are sure the drive letter is available on the pc that generates the error, it might help if you provide an example of what's in the control when that error is raised.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.