On occasion, the Mac hard disk can become confused as to who has what permissions to do what and where. There is a utility that can generally remedy this situation. Proceed as follows:
Depending on the speed of your processor, the size of your hard drive, and the number of files that you have stored, this process can take several minutes to complete. Once this task is completed, attempt to install the software again or to reconnect the USB device. In virtually all cases, this repair does the trick.