if using the USB to flash the box then NO, if you leave the usb stick in the box when you reboot the box it will flash again.
If you mean the usb used for recording/timeshift etc then yes keep it connected, you can replace it with the hdd but then leave the hdd connected.
Sorry for jumping on the thread but I have the same sort of question ? For recordings you can point the settings to the HDD, and for EPG, TIMESHIFT ETC you point the settings to usb storage?no, you should always have a usb initialised and mounted on the box, although the box will work, if you have a usb stick things such as timeshift, epg data and picons are stored on it. without a usb stick they would be stored in internal memory which isn't very large and if you add things such as IPTV you will probably find there is not enough space on internal memory for the iptv epg etc.
Okay will doJust put picons and epg on the HDD. They are stored in ram when the box starts up anyway so the HDD is not in constant use unless recording/timeshift.