List all disks:
$ diskutil list
[...]
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *8.0 GB disk2
Change permission on USB disk:
chown user /dev/disk2
Create virtualbox raw device:
sudo VBoxManage internalcommands createrawvmdk -filename ./usbdisk1.vmdk -rawdisk /dev/disk2
sudo chown user ./usbdisk1.vmdk
Finally, add the disk under virtualbox.