It's a general purpose landfill for a town of 150,000 people and according to the original article he only understood his loss three months after disposal, in which time a further 10,000 tonnes of rubbish had been deposited. That small HDD will be surrounded by much larger items: not just whatever can be crammed into a wheelie bin but also possibly larger special items such as bedframes, furniture, etc. That "goblin" will have to be able to tunnel through compacted masses of steel and concrete while being delicate enough not to destroy the target, and definitely needs to avoid breaching the landfill lining. Probably best not to count on the stability of the tunnel it's making, so it will need autonomous power. Probably easier to go for the moon landing X-prize.