I made some test to trying to reproduce that issue on OSX. It quiet difficult to figuring out which application cause that problem.
I found a workaround which most likely solve that issue.
It seems if the user force to close all of the running apps on the MAC that helps to free up the USB driver in this cause the updater starting properly.
Please follow this instructions:
1. Please find and start the “Automator” app in your launchpad.
2. Choose the “Application” in the Automator
3. In the search field type “quit” then select “Quit All Applications”
4. Make sure the “Ask to save changes” checkbox selected
5. Command + S –> or “File” -> “Save” name your application “Quit all applications”
6. Run that Quit all apps then start the USB updater