I couldn't get Preview to work in such cases either and stopped using it. The Transformation itself works fine as long as parameters are properly mapped to the Global Variables.

>I have the following SQL statement as source in my Transform Data Task...
>select * from dbo.BWR_GETINFO_RESULTS where cFileId = ?
>
>and in the DTS Package Properties, my input global variable is assigned like this...
>name is fileid
>type is string
>value is {4EA8FB8B-09FB-43A5-99C0-B767C73C3D30}
>
>also in the Parameters Mapping of the Transform Data Task...
>the Parameter 1 is set to fileid
>
>Everything looks fine but when I click on the preview button I got this...
>Error Source : Microsoft OLE DB Provider for SQL Server
>Error Description : No value given for one or more required parameters.
>
>It seems the global variable is ignored by the SQL execution.
>Someone can explain the cause and an other way to do?