I have installed Ubuntu to my usb disk. And later try to open ubuntu on usb disk from vmware. Vmware could not boot from usb disk. So I attach my usb disk physically to Vmware guest.
in the hardware tab, delete hard disk
choose hard disk
use physical disk (for advanced user) click next
in the device tab select your usb disk, it is physicaldrive3 for me. You can identify with disconnecting and trying it again.
click next and boot your machine