I don't make any image. This is the Linux way. When you plug pendrive into linux machine it shows as /dev/sdx
(where 'x' is {a,b,...}) so I actually mount device not an image. What I'm doing is just make any one (in this VirtualBox) can write and read from it and generate vmdk file for it (It's little file some KB, where my pendrive is 8GB). Anyway, I do it just once. As long as my pendrive is showing in my system as /dev/sdb I don't have to generate new vmdk file. Just make changes on pendrive and run VirtualBox.
One thing when I make changes ofcourse I have to mount somewhere my device(or hal do it for me),but when I want to boot form it in Virtualbox I have to unmount it. And that's it.
I hope now it's more clear and sorry for my English it's not my native language