By default ESXi 5 creates a GUID partition table (GPT) which requires the host to support booting from this. If the host doesn't support it, you may reinstall (i.e. wiping everything on the disk) ESXi with an MBR partition table. To do this start the installation and press Shift-O on the boot screen, then append "formatwithmbr" to the command line. See e.g. http://vmtoday.com/2012/09/esxi-5-wont-boot-from-usb/
André