virtualbox.org

It would be useful if you could ad a disk partition or even an entire disk as external memory for the VM so you don't have to use vdi anymore,like VMware. What I'm trying to do is a bootable USB flash(it's seen as an external usb hdisk not a removable) and I want to test it under a VM so I don't have to reboot every time my PC. Right now I'm doing it with VMware ...

VirtualBox does support vmdk raw host disk access, see section 9.10 of the manual. Is that you mean, or something else?

Nope .. that's not what I meant. In VMware you have 3 options when you want to add a new hard disk for your virtual machine. First: create a new disk image, Second: use an existing disk image, Third: use an physical disk (a real hard drive, not a image file with ext .vdi or .vmdk or .vhd ..etc). The third option is missing in VirtualBox ...
I hope it's much clearly now.

The third option is not missing. VMWare creates a VMDK image for that physical hard drive in the same way you have to do with VB on the command line.

No .. WMware with the third option use directly physical disk, it modifies master boot sector, partition boot sector ... of the physical disk. I've done this before.
For example .. i use my usb flash drive as a hard disk for virtual machine, I install an OS (portable), i remove usb and insert it in another computer and I'm able to boot on it from bios.
Is it clear now ?

That's the same as VB's RAW Disk access for the whole disk. Did you read the manual?

Yes, accessing physical disk can be done by command prompt. Check this post about Access Physical Disk in VirtualBox

Sorry .. my bad.