Doesn't your cable provider send the NTSC channels as clear QAM simultaneously?
Right off hand I don't know of any that will do all 3 'flavors (clear QAM/ATSC/NTSC)
There's only one I know of that uses USB 'storage' and that's only ATSC - Mediasonic HomeWorX HW150PVR with clear QAM via unsupported firmware.