DH's comparisson of this device and what maks a "true" device of this nature with the metric of TNG's PADD is... silly. And, IIRC, even PADDs had removable storage in the form of the Isolinear Chips. But even of they didn't they had the advantage of being "networked" with the Enterprise computer so anywhere on the ship, and presumably anywhere in the Federation, a person with a PADD could accsess their personal files. With the iPad, saving an internet file-storage service there's no way to get a personal file from your PC to the iPad (that I am aware of.)
So if the HP "PADD" can have a flash-drive connected to it that gives it a huge advantage over the iPad, a person can move his files from the computer to the "PADD" without having to go through the extra step of putting the file onto a web service and then
download it with the iPad.
And, where is it said that devices like these can't have USB ports, headphone jacks or things of that nature?