Print-Tool on the Mac can open both files with no problems. So this is clearly only a problem on Windows using QTRGui. My feeling at this point is that QTRGui is running into some kind of internal memory management issue when opening large TIF files. But I'd still like to know if there is any kind of "documented" (but maybe not largely known) limit on the maximum TIF file size in either QTRGui or Print-Tool.