Since GTA Underground is a mod, standard GTA San Andreas saves will not work and will likely crash the game. You must find a save specifically made for the version of GTA Underground you are running (e.g., Snapshot 3.2 or 4). Common Sources: GTA Underground ModDB page or specialized GTA save repositories like GTA-Savegames Version Match:

Even with a "portable" save, problems can arise. Here is the fix guide:

– wait 10-15 seconds for all scripts to initialize (Liberty City traffic, Vice City boats, new gang zones). If you see "Mission Passed!" spam or missing textures, the save is not compatible with your UG version.