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Bug when import in append mode.
The bug arises when attempting to import new rows using append mode. If there is a field with a non-null constraint the import process checks the staging table to see whether there are any rows with null values in for the field. If there are then the import
Unfortunately the Sql to count rows with null values is missing a where clause so there are false positives being generated.
See CountNullsInStaging() in SqlServerImportExportInterfaceImplementation.cs.
*** Workaround ***
Select Merge mode.