In iMovie on your SSD go to the Vie Menu and choose Group Events by Disk
That should allow you to see the other disk and iMovie should list the folders there. NOTE: But the issue I think is right now,
You have Two Boot Disks and that's a problem
Here's why. iMovie allows you to install and run it on your boot disk, but you have 2 of them now. Right? When iMovie let's you save your iMovie Events/Projects to a second HDD, it creates the folders at the Top Level of that hard drive. For instance I keep a bunch of iMovie Events/Projects on my Time Machine backup drive. It created those folders when I first copied or moved those items within iMovie to that other drive, and they are at the Top Level.
Now in your situation with 2 boot disks, you have 2 home folders with 2 Movies folders with 2 iMovie Projects folders, etc. The path to these folders is like this: /Users/HomeFolder/Movies/iMovies..... So when you are running iMovie on one of the boot disks it sees everything fine where it expects to see it on its own Boot Disk. But it will never see those folders EMBEDDED within the other boot disk. So here's my advice.
Try moving your iMovie Folders to the top level of the HDD disk, just drag 'em on up to the root level of the HDD. Restart iMovie, don't forget to Group Events by Disk and see if iMovie starts seeing your iMovie Events/Projects folders on the HDD. It's worth a shot.