You seriously can't imagine this or are you just being snarky? Happens to me all the time. Swap out hard drive to upgrade, throw old drive in box with other hardware junk, throw out junk box months later or when moving to new apartment or simply forget which old box you put that old hard drive in or even which old hard drive had those files etc. Even easier with usb hard drives or external drives.
Seriously, I lose stuff occasionally but nothing as bulky and "expensive" as a HDD. I got a box where I put all computer hardware and I think I just "mentally track" items that cost more than say 50$ and that I walk around with, so I just check for them like I check for my keys or mobile when I leave the house.
Now a pen-drive maybe, but that wouldn't be the place to store my wallet anyway.
(Sounds like he went to a good party, the night was long and when he finally got back home he realized he didn't have his hard drive on him anymore.)