Mount a disk image as physical USB drive? I have an certificate manager ("BankID") that allows me to export certificates from its certificate store. However, for some stupid reason it only allows me to export these certificates to USB drives, not to a local file system, or, as I want to, a cloud storage unit with file system integration (I use Wuala which use MacFuse for its file system integration).

Is there a way to make my Wuala volume behave and look like a USB drive?
I am not familiar with this particular certificate manager, but generally I would suggest to create an empty disk image; I hope this will work for you.

Open "Disk Utility" and simply click on "New Image" and save it to your hard drive. Now, if you mount the disk image (double click on it) you can save data to it.