The files generated by the Phase One backs have the extension TIF, though they are not TIFFs. They require raw conversion software to open. All that you can otherwise see is the embedded JPG thumbnail.
That's why we included the free trial copy of Capture One software for both Windows and Macs on the disk. So that these files could be opened properly.
Ps: Some people responded to this query by confusing it with a problem that existed on the forum with embedded files. I have deleted those messages because they just will create greater confusion.