James Howells, 28, obtained 7,500 Bitcoins in 2009 when the currency was virtually worthless. Its value has since soared, with a single Bitcoin hitting $1,000 for the first time on Tuesday.
Howells left the hard drive in a drawer for several years, and threw it away earlier this year without a second thought.
To his horror, he then realised what it had contained and that he would be a millionaire, if he could only find it.
He now faces the prospect of a painstaking hunt through a massive landfill site in his home city of Newport, Wales, he told BBC television on Thursday.
"When I went to the tip the manager took me up to the current landfill site and when I saw it it's about the size of a football field my first thought was 'no chance'," he said.
"The manager explained that things that were sent to landfill three or four months ago could be three to five feet (1.5 metres) deep.
"He confirmed my worst fears when he said that."
Large-scale searches of the landfill site including those for police evidence usually involve up to 20 workers with digging equipment and dogs, he added.
"The truth is, I haven't got the funds or ability to make that happen at the moment without a definite pay cheque at the end of it," Howells said.
He told the BBC he forgot about the Bitcoin stash because he was "distracted by family life and moving house".
The hard drive was thrown out between mid-June and August, he believes.
He has checked all of his back-up files without success.
Launched in 2009 as the invention of a mysterious computer guru who goes by the pseudonym Satoshi Nakamoto, Bitcoins can be exchanged online for real money or used to buy goods and services on the Internet.
The currency is not regulated by any government.
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