I had the same problem with VMWare Fusion on the Mac. I figured out a solution.
Make sure that your settings are set to auto connect USB devices. Physically remove the drive from your mac. Start the virtual machine. Select USB, then immediately after, plug your USB drive back in to the computer. An error should come up. Hit enter to return to the menu. Select USB again, and this time, it should boot into GRUB. It's worked for me.