How do I "get permission" to save to an external drive?

I am attempting to save files from FCPX to an external drive, something I've done countless times in the past. Suddenly now though I'm getting a warning stating that I don't have permission to do so. I am directed to select the item in the folder and then select "Get info". I've done this repeatedly to no avail. Can anyone suggest a fix? The weird thing is that if I save the file to my desktop I can then drag it right into the external drive, where I actually want it, and that works just fine. I just can't do this directly from FCPX, and have not had this issue elsewhere with any other app.