Error: Multiple projects found in the XER File. When There Should be Only One Schedule per Update

When uploading P6 schedules to SmartPM the .xer file cannot have multiple schedules in it.

This error message is specific to P6 files because P6 allows you to roll up multiple schedules into a single .xer file, whereas Microsoft Project does not have this feature.  This video walks through breaking out multiple .xer files, as well as viewing and deleting phantom schedules.