You can't boot a VM from USB devices. Full stop.
You can attach the USB-disk as a physical disk to the VM - then it should be possible.
But ... a setup like that has no advantages compared with a regular installation into a virtual disk - it only has much more problems
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description of vmx-parameters: http://sanbarrow.com/vmx.html
VMware-liveCD: http://sanbarrow.com/moa.html
