What the heck??? I've always done it this way when putting a copy of our Revit file on our FTP for consultants. While my local file is still open, I open the Central file with the Detach option checked, and then save it as a new file on our FTP site. Now it's not allowing me to do so. So I have to close out of my local file just to open a detached version of the Central file?? What a pain in the rear! Really puts a hamper on our workflow.
I understand not allowing the Central to be opened at the same time as the Local (although I have done this before, also, to copy/paste back in elements that were inadvertently deleted), but it really doesn't make sense to not allow a detached version to be opened at the same time.