I just installed mine last night //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif been using the SD card and has worked flawlessly.
I read about a guy who successfully used an external on the 4200 (previous model). It must be in FAT32 and the HU must be able to power it through the USB port alone of course, so a huge drive would probly be less likely to work. I will test this as soon as I can and update the review once I do //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif