>>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?