@InfernalDiva What's the file path for the game's installed files? You can right-click on the "The Sims 4" folder on your external and choose Properties to see it. Please also let me know whether Steam is also installed on the external drive.
Do you have more than one external device that gets assigned a letter (e.g. a USB stick), and if so, do you always plug in the devices in the same order? Windows assigns drive letters in the order in which the devices are connected in that session, and if the external winds up with a different letter than it had when you installed, that could prevent the install process, or the game for that matter, from working correctly.
@InfernalDiva Try uninstalling Sims 4, and Steam as well, and reinstalling them into a (different) folder on D. So you'd install Steam into, for example, D:\Games, and Sims 4 would end up in D:\Games\Steam\steamapps\common. Let me know whether any packs show up (those from Steam or Origin).
