Thanks for your answer.Actually I had looked at that webpage before, it doesn't say something like "internal hdd" or "Detachable HDD" so I think Uno has neither an internal hdd nor "detachable hdd" tray.It's interesting that when I downloaded their manuals, at least in one place it uses the term "internal hdd" like the device has it though it doesn't have.
From my search on Google about the subject yesterday, I have the impression that Vu+ Uno 4k doesn't have internal hdd, but it has usb ports and supports recording on a usb hdd using a third-party image, though not with stock image.
Zero 4k has an optional pvr kit so I can use a sata hdd, but I am not sure if it supports recording to usb hdd using a third-party image or not...