I have no idea. So far I was using the front USB port to put the new firms. But the on last one the image was not recognized so I just tried it with the back one and it worked. Before I used to plug 2 usb flash drives in both ports for firm and backup settings but now I use the HDD for backup.
For me it is always pain in the ass putting a new firm as the plugins are never transferred correctly. At least the sat/channel/bouquets data is OK. The rest I do it manually.
