Tools required to do the job:

• At least 1G USB flash drive
• Virtual CD mounting tool, i.e. MagicDisk
• WinImage to build the image to USB storage.

Steps to make the bootable ESX Server 3i USB flash drive:

1. Download ESXi Installable ISO
2. For 4.0 extract INSTALL.TGZ from the root directory of the ISO image using WINIMAGE the Virtual CD mounting tool
   For 4.1 extract imagedd.bz2 from the root off the ISO to your harddisk using WINIMAGE. (WinRAR gave me problems later on the process)
3. For 4.0 extract /usr/lib/vmware/installer/VMware-VMvisor-big-3.5.0-xxxxx.i386.dd.bz2 from INSTALL.TGZ
4. For 4.0 extract VMware-VMvisor-big-3.5.0-xxxxx.i386.dd from VMware-VMvisor-big-3.5.0-xxxxx.i386.dd.bz2
   For 4.1 extract imagedd form imagedd.bz2
5. Attach the USB flash drive and make sure you no longer need the data on it
6. Use WinImage to transfer VMware-VMvisor-big-3.5.0-xxxxx.i386.dd to the USB flash drive
   1. Disk->Restore Virtual Hard Disk image on physical drive…
   2. Select the USB flash drive (Warning: If you select the wrong disk you will lose data!)
   3. Select the image file VMware-VMvisor-big-3.5.0-xxxxx.i386.dd or imagedd
   4. Confirm the warning message
   5. Wait for the transfer to complete
7. Unplug the USB flash drive (Warning: If you forget to unplug the flash drive from the PC you might lose the data on your hard drives the next time you boot!)
8. Attach the USB flash drive to the machine you want to boot (Warning: If ESX Server 3i recognizes local drives, you might lose the data on it, so make sure you don´t need it anymore or unplug all hard drives!)
9. Turn the machine on and make sure the USB flash drive is selected as boot device
10. Enjoy.

Notes:

The physical machine you want to boot from must have the option to make the USB flash drive as the boot device.

You can also watch a tutorial video.