Is this for the XAV-AX100? If so, please try again using another USB device and press the Reset button. You can find the full firmware update instructions here.
As described in another forum to solve the problem (or avoid it) use a usb 2.0 stick with a capacity of 4 Gb. Format it using FAT32 an assign a name (whatever you like). Using this trick the endless loop will end and the update will work perfectly.
