Create a "Virtual memory card/camera" from folder?

Is it possible to create a virtual memory card from a folder's contents or an external hard drive? In other words, make the Mac see a folder or external hard drive as a memory card or attached camera.

I use Adobe's "Photo downloader" (part of Adobe Bridge) for importing photos from my camera memory card, but occasionaly I need to import photos
but occasionaly I have images stored on other media which I want to import (typically a folder on my hard drive or an external hard drive) and "Photo downloader" refuses to access anything but memory cards or attached cameras.

 

I tried your suggestion but had none of your luck. Perhaps I didn't quite understand what you meant, but I did enter your listed commands in the OSX Terminal which indeed created a RAMdisk (which unfortunately didn't show up in the Photo Downloader). Then it occured to me that my Powerbook only has 2GB of RAM, which really is nothing (memory cards I have are 8GB!).

But all of this got my mind going, so I tried creating a disk image and it worked!
Here's what I did:

1. Run "Disk utility", then select the File-New-Blank disk image menu
2. A new window pops up. Choose any descriptive name for the disk image itself, as for the volume name I entered "CANON_DC" (as my camera is a Canon and that's the name that pops up on my desktop when I insert the memory card into the reader). 8GB as the volume size, as that's the size of my memory card, MS-DOS as the volume format (doing a Finder "Get info" on the memory card reveals that MS-DOS is used). I suppose other camera's memory cards might have different names etc. Here's a screenshot of what I did:
   
3. Disk Utility then proceeds to create the disk image and finally mounts a white "CANON_DC" icon on the desktop just as if I had a memory card inserted into my card reader.
4. Double-clicking the "CANON_DC" icon naturally reveals that it's empty, so now is the time to fill it up with images.
5. On my hard drive I've copied numerous memory card dumps into new folders. They all contain a folder named "DCIM" (with a sub-folder containing the images) and sometimes also another folder (at the same root level) named "MISC". In any case I just copy everything over to the "CANON_DC" on my desktop. For some reason this is really slow, but it might be partly because I have an old computer (Powerbook G4).
6. I should now have an icon on my desktop which looks exactly like a Canon memory card and its contents exactly the same as a Canon memory card with images.
7. It's time to import the virtual memory card, so I run Adobe "Photo Downloader" and presto! the virtual memory card appears and can be imported just like a real memory card.

It's probably a good idea to create several differently sized "memory card" disk images, similar to the sizes of the "real" memory cards you have. I don't know, but smaller memory cards disk images might respond quicker when you fill them up (provided there's enough space available for your files).
Give it a go and tell me if you get it working as well.