If you are going to install VMWare ESX/ESXi on a USB flash drive, my suggestion is to do the install on the final destination machine. Don't install it on one machine, and then move it to another. I'd love to give you the long, and in hindsight amusing, story of how I discovered this nugget of knowledge; but I'm tired and need to sleep.
Te default settings for VMWare ESXi are to clone the MAC address of the management network interface and use the cloned address for the virtual management NIC. While VMWare also supports updating that MAC address when the NIC changes, that feature is disabled by default.
I wound up with two servers in the cage that had the same MAC address. Boy, that was fun.
Figuring out that the problem was duplicate MAC addresses on the network was fairly easy. Figuring out why two SuperMicro motherboards had onboard NICs reporting the same MAC address took a little longer.