Save existing project to an external drive Final Cut Pro X 10
"I copy pasted Final Cut Events and Final Cut Projects folders to my external HD but I don't know how to restore it into project in FC. Any help is appreciated."

It's better to perform this type of operation from within FCPX, instead of the Finder via copy/paste.  With the project selected in the Project Library, choose File->Move Project...  As long as you have an external drive connected, the resultant "location" dropdown menu will be populated (instead of saying "no value").  Selecting the "Move project and referenced events" radio button will transfer all of the necessary media to the external drive, AND remove it from the local drive, thus freeing up space.

When you work this way, there is no need to, as you say, "restore it to a project in FC".  You simply make sure the external drive which contains the project file and media is connected to your machine, and the project is available in the project library window.