1. Open a new, empty PP project.
2. Go to Edit>Preferences>Media...
3. Take note at the top where the Media Cache files are stored.
4. Below that, Clean the Media Cache Database.
5. Close PP, go the location you noted from step 3 and delete those files.
6. Reopen your project and let PP rebuild.
Welcome to the forum.
When working with external HDD's, the biggest problem is maintaining the drive letter (Windows), or voume name (Mac). One must set those to a perminent drive letter, or volume. This article goes into more detail: http://forums.adobe.com/message/4542396#4542396
The "Where is file ____ ?" is telling you that the absolute Path to the files has been broken. The previous article addresses one of the prime reasons for that: http://forums.adobe.com/message/4598705#4598705
Those are for background, and Jim has addressed getting your Project back up and running.
Thank you very much! Both answers has been to great help. I followed your instructions, Jim, and the project is back up and running smoothly Learned a lot from your links Bill! Thanks again