Which exact error message is coming up, while you use start the app "digital recording"?
"To connect a dedicated DVR USB storage device
1 Connect a USB HDD (40 GB or larger) to the HDD IN jack on the TV. The DVR
function works only with the HDD IN jack.
2 USB HDD must be initialized to be used for DVR.
When connecting a USB HDD that has not been initialized, select MY APPS ➾
DVR. USB Device Initialization screen appears.
3 Complete the USB HDD initialization, and set Timeshift Mode to On.
✎✎To remove a USB storage device, select Quick Menu ➙ USB Device and
confirm the message that the USB storage device has been removed before
detaching it physically." source LG.com
Did the recording work in the past?
Which exact HDD brand and model is plugged into the HDD port of the TV?
Try all 3 USB ports as well.
Is the updated firmware/software, version 04.62.12 on the TV?
Can you show pictures/play videos from the HDD?
Set the "HBB TV" feature to inactive in the menu.
power off the TV, remove the power cable, press the power button at the TV for about 30seconds, power on again