How do I install Mavericks onto external HD but from inside VirtualBox
First make sure your external HD is attached. Then find out what device node your HD is. In your case it's /dev/disk2. Umount it with

diskutil umountDisk disk2


and wait until it disappeared from Finder. Change the owner of the device node to your user:

sudo chown `whoami` /dev/disk2


Then create the vmdk:

sudo VBoxManage internalcommands createrawvmdk -filename ~/disk2.vmdk -rawdisk /dev/disk2


Right then attach it to your VM and start it. If you unplug the drive, the ownership and permissions get reset, so you'll have to do the chown step again.