Another option you could use if you just want to copy from one project to another would be to use the Migrate tool. If you right-click on an asset that you want to copy into another project and select Migrate, you will be presented with a list of all the files that need to be moved for that asset. After clicking OK, you will be able to navigate to another project’s Content folder and select it as your destination by clicking OK once again. After copying, you should get a confirmation message that all of the files were moved.
-Steve