Yes I've tried from both USB drives, one is 4GB the other is 32GB.
Both keys are bootable, I was able to get the ESXi installer to boot and load on my home PC and my Macbook.
The USB ports on the servers are working as well because I've plugged the keyboard into all of them to test and the BIOS detects the flash drives as "Hi Speed USB Generic" and USB Mass storage device.