If you only added the PROJECT MEDIA folder manually, then M100 might still be looking for the RENDERED MEDIA folder which is a sub-folder of the above.
Also, there’s an option under MEDIA 100 PREFERENCES for MEDIA searches. If one of the options there is turned off, M100 might not be looking in the external drive when you do the auto search.
I prefer to tell M100 manually where to look as the exhaustive searches tend to take a long time.
Jaeson