Moving the Game to a Different Disk Drive
If you just copy the game to a new drive, things go horribly wrong - especially with mods.
Correct Procedure:
Backup your save games
Usually found at
C:\Users\<your_username>\AppData\Local\Colossal Order\Cities_Skylines\Saves
on WindowsAlternatively, you can save them to Steam Cloud, but many users say it's unreliable :/
Follow this guide: Moving a Steam Installation and Games
This will ensure you properly move Steam and Games to a new drive
Simply copying stuff over manually won't work, you have to do the correct procedure linked above
Reinstall Cities: Skylines
Steam will automatically download any DLCs and workshop mods/assets you were subscribed to
You will need to re-enable mods and check their settings
Restore your saved games (Steam will automatically restore any that you stored in Steam Cloud, assuming it works)