virtualbox.org

If I create a VirtualBox VDI file on an USB, can I use it on different platform, such Mac, PC, or Linux?

In theory, yes. But why not simply use Shared Folders or the USB itself directly?

I think he means "can I have a physically-portable OS", not just the data. And yes, it works fine (for Windows and Linux, at least) - just make sure the USB is formatted to something that all the hosts can read and write (i.e. "don't use extN for it").