you don’t need to add anything to the network path as you want to use a HDD directly connected to the Pi. There is an option to make the mountpoint of the HDD fixed, in case the mount folder changes after restart. Normally you don’t need to add anything to the mountpoints manually for a local connected HDD. The mountpoints are meant to be for network connections (NAS / other computer).
But the problem, that the HDD is not shown in the External Drives section is strange. Maybe something in the configuration file is messed up – but after burning a new Image this should not be a problem.
If you never see the HDD mounted or available in the External Drives section, you might consider removing anything but the HDD. Use a new SD-Card-Image and Ethernet via cable. Just to rule out a problem with power or interfering devices.
One more idea: start the pi without the HDD connected and wait a minute – then connect the HDD and check if this shows anything besides „No External Devices Found“.