Well after a couple of hours of pulling my hair and trying to figure it all out ‘I THINK’ I found an answer.
My first three projects didn’t have compound clips in them whereas the last one did. I’m guessing since creating a compound clip does something different in the Events folder, the program decides that the Events folder on the Main computer is different than the one on the outsourced file, therefore the outsourced project can’t reference the Main computer Event, even when they have the exact same media.
To test this, I erased the compound clips from the outsourced project, then I clicked “Modify Reference Event” and it let me choose my Main Drive Event just like it had with the 3 projects before.