Sean, I know you're busy so any help you give me is appreciated. It doesn't go with the territory that you have to support something you gave me as a sample but maybe you're intrigued enough to at least want to discover what the problem is.

I have tried so many things. The latest attempt was to try the code on a smaller sample of data. So I named a table with only FUSION I4 SE cars as "VehicleDetail" and renamed the original table as VehicleDetailTemp.

I ran your original code only changing the following (new line boldface):

Update VehiclesToUpdate

Set NewReturnDate = ReturnDate

Where ReturnDate Is Null

There were 3728 records with a null return date. Only 683 of them actually updated. The funny thing is that VehiclesToUpdate is 3728 before the update occurs. So why would the update query be selective? Is it because of duplicate records in VehiclesToUpdate?

I think your idea about sending you a data set is great. However, isn't there a limit as to how many records can be sent? What size dataset would you need?