I do not have any plans to install an optical drive in this server but I still need to install ESXi from an ISO image to a USB drive. This is one way to do it:

1. Fire up your favorite virtual environment that can handle the following requirements
   • Assign 2 processor cores to a VM
   • Assign 3GB of virtual RAM to the VM
   • Assign an ESXi supported network controller to the VM
   • Enable hardware virtualization to the VM

   Here are some screen shots from VirtualBox running under Ubuntu 12.04:
   
   
   
   

   Load the ESXi ISO image to the virtual CD drive
   

   An overview of all settings
   

2. Insert the target USB drive into the host
3. Launch the VM and press a key to halt the ESXi installer at the following screen
   
   Right click in the USB icon next to the CDROM icon and select the USB drive.
4. Finalize the ESXi installation and shutdown the virtual machine.
5. Move the USB drive to the physical machine and verify that it boots.
   Note: The physical machine must be able to boot off a drive with a GPT. Look for a UEFI Boot settings in BIOS.