I have the same Problem with the LG UN7300 from italy. The reason for this is, that LG doesn't pay the italian tax for recording devices (SIAE), which also includes TV with DVR/PVR.
I went in the EZ-ADJUST menu and found the DVR Ready function under "ToolOPT6_Energy/Country".
It's greyed out so it's not possible to turn it on.
Would it be possible to change the number of "ToolOPT6_Energy/Country" to a number for a country where USB Recording is available? Is someone here with an LG UN7300 and working Recording who can share this number?