I have a new external drive with .mov, .wav and .m4a clips on it. A total of about 25. When I go to the source browser screen I only see items marked "Clip 1" "Clip2" etc and there are only six of them. And the Plug-In column says can't be linked.

I discovered that I can drag and drop the files directly into the bin but that will be extremely time consuming and I also don't want this to be an ongoing problem.

In the software, select all the clips in the “offline” bin → right click → Relink to Media / Link Media / Batch Import (depending on the geometry dash lite software). In the dialog box that appears, navigate to the exact folder on the external drive that contains the original file.