by moving the Aperture library, will that cause issues at all when I reopen the program? Will it have to re-index all of the files
That is a question for the guys over in the Aperture forum. I've done this with iPhoto libraries and there are no issues at all, but I have not used Aperture.
I imagine that you will have to "point" the Aperture application to the "new" location of the library on the external hard drive once it has been re-formatted and the library has been moved back over.
At least that is the way that it works with iPhoto....you hold down the option key on your Mac while starting the iPhoto application and simply point to the location of the library. After that, iPhoto knows where the library is located and will open it automatically when the application starts.