>>>Can you post what you tried already?
>>>
>>>To me this sounds like
>>>
>>>trigger body
>>>
>>>insert into mySecondTable select ID, some other info from Inserted
>>
>>This works - but requires a ProductFileId field in ProductFileData:

    DECLARE @rowid uniqueidentifier
>>    SET @rowid = NEWID()
>>    INSERT INTO ProductFileData
>>    (ProductFileColumnId,ProductFileRowId,ProductFileId)
>>    SELECT INSERTED.Id,@rowid,INSERTED.ProductFileId
>>    FROM INSERTED INNER JOIN ProductFileData ON ProductFileData.ProductFileId = INSERTED.ProductFileId

>>
>>UPDATE: On second thoughts - it doesn't - I need unique row IDs.....
>
>Exactly - I was going to tell that it's not going to work this way because of multiple inserts possibility.
>
>But would this work?
>

>    INSERT INTO ProductFileData
>    (ProductFileColumnId,ProductFileRowId,ProductFileId)
>    SELECT INSERTED.Id,NewID(),INSERTED.ProductFileId
>    FROM INSERTED INNER JOIN ProductFileData ON ProductFileData.ProductFileId = INSERTED.ProductFileId

>
>Or why not let ProductFileRowID field to have default NewID() and then you don't need to include it in insert?

Actually what I posted above is complete rubbish. The ProductFileRowId has to be based on existing unique values for existing rows.
Using NEWID() is completely wrong. Using a default would be worse because all the rows would have different Ids. Let me try again....