So you want the laptop's HDD to look like a USB memory device to anything that plugs into its USB port via a cable?
Well I can't say I've heard of anyone doing as you suggest. You'd need to have some software on the laptop send all the USB ID information that USB devices identify themselves as. It would probably be best for it to identify itself as an external USB HDD, so that the accessing device can accept that it behaves like a HDD and probably doesn't have a FAT file system.
You would have to have the "USB HDD Emulator" software on the laptop access the HDD data directly - like formatting and partitioning programs do. Of course you certainly couldn't have the drive mounted by the laptop at the same time as it was mounted as a USB device by another machine, two systems managing the same file system would conflict horribly (unless there's some file system format that allows for it).
You would have to use a USB crossover cable as well, these are fairly easy to buy.
So it's most likely possible, but I don't know of any software to do the job for you (I haven't looked though, so you should do some decent searching). Certainly if there's any way to do the same job over a network using FTP, a remote file system format, or a USB file transfer system, it would be far simpler than this.