Q: fcpx external drive library clips disappear

First time FCPX user. 

I run a late 2012 iMac and was using iMovie 10.1.2 until yesterday.  Just before purchasing FCPX, I ran out of hard drive space and got a 4TB external drive and transferred my iMovie library there.  I renamed the iMovie library “external iMovie library” and deleted the old one on my Mac, freeing up 500Gb of space.  Tested it with iMovie using the external drive, no problem, everything was running ok.

Then I purchased FCPX yesterday.  To get access to all my old imovie clips, I used iMovie and opened "external iMovie library” and used the “send library to Final Cut Pro” command from the iMovie file menu.  All looked good, as I was able to open the library in FCPX and all my clips and events were there.

I thought everything was great, so I proceeded to add some more clips to FCPX from my video camera.  I created a second FCPX library and put the clips there.  They downloaded fully, but after the download said it had finished, my machine froze and I had to do a hard reboot by turning off the power.  This isn’t all that unusual, sometimes my Mac freezes when I leave it alone for several hours.

After I powered up and opened FCPX, I see in the left pane the two libraries and the events, but no clips show up on my screen.  I’ve opened and closed the libraries hoping to get the clips back, without success.

Any idea what went wrong or how to fix it?  I don’t know whether to attribute the problem to the computer freezing, or instead to using for the first time an external drive to house my files.

If I can’t figure out what went wrong, then I will simply repeat the process I went through the first time and see whether I get the same results, but I’d prefer to understand what I did wrong and try to fix it so I have a higher comfort level moving forward.   I’m hoping that this is not going to be a repeat experience just because I’m hosting my video files on an external drive.

Thank you!