If you could mount it with the mount command, the drive is likely physically fine. My guess would be that something in PopOS didn’t mount the filesystem correctly. I’m not sure how PopOS handles automatically mounting drives. If it were a drive that was always connected, you could tell PopOS to mount it on every boot by putting the correct line in /etc/fstab
To take this thought to the logical maximum, we are all under the stars, always. All buildings are ultimately outside, and therefore under the stars. Even in the sub-basement of the tallest building, the floors above aren’t that high compared to the atmosphere, and it’s still under the stars.
Unless the animals have developed a way to move for miles every day, there should be predators who are adjusted for the night side, and predators who are adjusted for the day side that would be well known and defended against from the prey on their side. For a sentient species, figuring out how to defend against one or the other shouldn’t be too hard.
What would be harder to defend against would be those predators who live in the twilight areas are close to both day & night.
In 6,000 year creationism, some sects believe there was a physical firmament (basically a shell of ice around the earth), but that it fell during Noah’s flood.