First: Do NOT import or re-import these photos. It will create trouble for you. Importing is only for photos that are not yet in the catalog.
Since your 300 photos are already in the catalog, import is not an option.
The correct way is: Since you changed the location of your photos outside of Lr you have to re-connect Lr to these folders.
In the folders panel (Library - left side) you should see question marks besides the folders. Right-click the top-most folder in the hierarchy of folders (with question marks) and select <Find missing folder>. Then navigate to the location of the folder on your external drive and click on it. Lr should then find all the sub-folders on its own, but only if you have not changed the folder names and/or folder structure since your last session with the trial version. If you did change folder names or structure you have to do the "Find missing folder" procedure for each folder.
If you also have moved single images to a different folder you will have to locate these photos individually. In Grid View you will see a question mark on the grey frame of these images. Left-click the question mark (you have to hit it directly) and select <Locate>. Then navigate to the correct location of this image and click on it.
Since Lr is sensitive to image location (image and folder location and names of folders and images are stored in the catalog) it is best practice to do all image management in Lr only.
