From the VirtualBox Documentation

9.9.1.1. Access to entire physical hard disk

In short:

$ vboxmanage internalcommands createrawvmdk -rawdisk '\\.\PhysicalDrive1' -filename usb.vmdk

RAW host disk access VMDK file usb.vmdk created successfully.

Connect it:

$ vboxmanage storageattach USBBoot --storagectl "IDE" --port 0 --device 0 --type hdd --medium 'c:\Users\joeb/VirtualBox VMs\USBBoot\usb.vmdk'

Note: The drive must not have any partitions on it otherwise VirtualBox will not be able to access it. Use diskmgmt.msc to delete all partitions and all volumes.