If you read around, you’ll discover that many have found that conversion with another program first before handing it over to SMC is critical for success. Some recommend AnyVideoConverter, others have their own favorites.
You might try this approach with your mp4 files.
Be sure to have the latest version of AnyVideoConverter. I see that they have made some improvements to the application, and it’s freeware too!
Had experienced it before and it has nothing to do with video formats or patches, it all has to do with permission. It happened on my Vista and now on Win7. You just change the SMC program properties (right click on the icon and choose properties and click the Compatibility tab) to 1; Run the program in compatibility mode for Windows XP and 2; Make sure you enable the program to run as Administrator under the Privilege Level. Good Luck to all you frustrated Sansa users.