How an I reconcile which records didn't get imported?

About 2 years into Sugar and I've discovered that not all our contacts were imported. The files were uploaded but something must have happened during import that caused some records not to get in there.

I have the spreadsheet of what was supposed to be uploaded and I exported what's in Sugar. How can I reconcile these two list that contain 10,000+ contacts in a way that shows me what didn't get uploaded?