Compare Data In 2 Columns Extract Unique Values And Paste Into 3rd Column?

Here's how my daily report is currently set up:

ExternalInternalDifferences
123000123000
234564234564
345456345456
456567378987
565456456567
654547524564
788879565456
865478654547
745654
788879
865478

The External column contains data provided by an external vendor. The Internal column contains data compiled internally. The data in the Internal column will always contain data duplicate to the External column, but will also contain unique data not present in the External column. I would normally cut out the data contained in Internal column that is not duplicated in the External column, and paste it into the Differences column, and move the remaining cells in the Internal column up. The result would be the External column data and Internal column data would align by row, and the data unique to the Internal column is segregated to the Differences column, like so:

ExternalInternalDifferences
123000123000378987
234564234564524564
345456345456745654
456567456567
565456565456
654547654547
788879788879
865478865478

While not a difficult task for 8 rows of data, the actual daily report contains over 1000 rows, on average. I would like to set up a template workbook, where I can simply open the template workbook, paste the data into both the External and Internal columns, then have the values that are unique to the Internal column extracted from the Internal column and inserted into the Differences column.

The end product would be used daily, with differing amounts of data (from 100 rows to 10,000 rows).