I have only seen this happen when the owner unplugged the drive quickly.
I recently transferred some folders/files (music and iso files) from my friend's laptop into my Toshiba external hard disk. But when I connect it to my laptop, those folders/files are not there. I've used my friend's laptop to transfer things into my hard drive before, and had no problems. All my previous files/folders etc are there and remained untouched; it's just those newly transferred files that are not there (as if they hadn't existed at all!). I opened the files on my hard disk through my friend's laptop just to make sure they're there and working, which they were. (I transferred them yesterday and found them gone when I used my laptop at home, so I initially assumed that I somehow hadn't transferred it properly)
The files show when I open them on my friend's laptop, but not on mine.
I've tried everything. The files aren't hidden (and I double-checked just to be sure), I've run cmd and attrib prompts (none worked), I've done an antivirus scan on the whole disk, and checked for damage. I'm just about desperate for answers.
Anyone know of any solution? I can still transfer things from my own laptop to the hard disk with no problem.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic