Aperture says it is merging my two libraries but nothing happens

I am trying to merge my two libraries. I go to File>Import>Library. Aperture asks me if I want to add or merge, I say merge. A a status bar says it's merging, then when it's "done", nothing has happened. I am sure nothing has happened because I'm importing a library from my external drive- to my library on my iMac, and no additional memory has been taken up after the libraries have supposedly merged. I would appreciate any insights. Thanks.