Same thing is also happening is Logic Pro X... but I have not seen or found links to the new Logic Pro X additional and basic content downloads.
ART OF NOISE helped with Logic 9 by offering up the following :
However, to resolve this... use these external links and download and then install all the essential and additional content instead.
I have a similar problem.
I bought Logic X - and it tells me it's downloading basic content - which takes 2 hours!
I then get a window saying the installer wants to make changes to the system, type in permission, installation appears to complete.
I then click on the App icon and it starts downloading basic content again!