IMovie Missing Projects & Media Using External Drive

Prelude:

I purchased a new MacBook Pro. My old one was still good but was aging and had a very dangerous swollen battery. I had the iMovie projects and media put on an external drive based on recommendations. (My previous MacBook Pro had 1 TB and all IMovie stuff on it.) At first, everything was there, and I had no issues. Then one day, iMovie started acting strangely. I could not share my projects as MP4 (which I normally do). Then, when I inserted the external drive and clicked on things, several of my projects and about half of media seemed to disappear. I was frustrated. More projects actually seemed to disappear as time went on.

To clarify, it showed they were still on the external drive but were not popping pop in the iMovie program. I did delete iMovie and re-installed it - twice as a matter of fact. Apple Support had me click on some things, but this did not help.

Temporary Solution?

This went on for about three frustrating weeks. One day, I was considering trying to transfer all of my media and projects from external drive back on to my Mac Pro as a desperation attempt. I actually went to Best Buy for assistance with the Geek Squad. Here's the kicker! Just after I inserted my external drive (with the girl ready to assist me), ALL of my projects and media miraculously showed up in the IMovie program. Although I felt stupid at that moment, I was just delighted, but had no idea why it or how it happened. The girl mentioned something about "background". This I had no clue about? I just knew those things were always there on the external drive. I did manage to make several important presentations and share then as MP4.

Reoccurring Issue!

Just recently (two months later), I was unable to share a simple project to MP4. I had a bad omen. Now, same issues as before. About half of my media and many projects don't appear on IMovie when using my external drive.

Does anybody have any idea what may be happening - many of my media and projects seem to be missing but show they are on the external drive? Could the SanDisk external drive bad? Best Buy says it's NOT the disk.

Is it an IMovie issue perhaps? Do I need to do things in my Preferences?

I just feel screaming "Give me my old MacBook back with my TB - swollen battery and all!" Yes, I've always backed it up on another external drive. This current external drive, despite recommendations by many Mac Book Pro users and YouTube videos, is not practical for me when using IMovie!

Should I transfer the external drive back on the MacBook Pro? I do have enough memory. I could always add more to avoid the hassles and unreliability of the external drive.

Although I'm retired, I do make many presentations about African wildlife to supplement my pension income + it's fun doing this. I depend on my IMovie. Can't have these issues! I really don't need Final Cut Pro.

Any assistance please? Thank you, so much Mac Book Pro and IMovie folks!