How do I install Mavericks onto external HD but from inside VirtualBox I don't want to lock down my machine for the install. Can I hand it off to a virtual box process? 


Open virtual box
Create virtual Mac
Attach Mavericks install pen drive to virtual Mac
Attach real external HD to virtual Mac
Boot virtual Mac from Mavericks pen drive
Install to real external HD from inside virtual Mac


Would/should/could this work?

I'm stuck at step 3.

I've entered:

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


which outputs:

VBoxManage: error: VMDK: could not open raw disk file '/dev/disk2'  
VBoxManage: error: Error code VERR_RESOURCE_BUSY at /Users/vbox/tinderbox/4.3-mac-rel/src/VBox/Storage/VMDK.cpp(3390) in function int vmdkCreateRawImage(VMDKIMAGE*, VBOXHDDRAW*, uint64_t)
VBoxManage: error: Cannot create the raw disk VMDK: VERR_RESOURCE_BUSY
VBoxManage: error: The raw disk vmdk file was not created
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.