Unfortunately, if you jut move the folders then re-import them the edits are lost unless you have saved them with the original file with an XMP sidecar.
To prevent this, there are three ways that I'm familiar with. The most integrated way is to move the files in Lightroom. You can import a main folder on your external drive then select an existing folder on your internal drive and then right-click on the external main drive and select to move the selected folder into this folder and it will do it, metadata and all.
Or, you can just move the folders outside of Lightroom. When you start up Lightroom, the mising folders/pictures will turn red. If you right-click on the red folder and select Find Missing Files or Folders Lightroon can locate them if the external hard drive is connected.
The third way is less desireable because it is incomplete. You can select files and, in the Library module, select the files in a folder and go to Metadata/Save Metadata to File and it will write an XMP metadata sidecar for each file. After you move the files you can import them, then with all the files selected you can choose Metadata/Read Metadata from file. Your edits will all be there but things like keywords won't.
Like I said, the third option is the least desireable for moving the files withing Lightroom. It does come in handy, though, when sharing files across multiple users or with and Adobe application (Bridge->ACR->Photoshop). Bridge can read the XMP files (it may need to be directed to do this).
Hope this helps!