With the release of VMware vSphere 4.x, VMware has made it possible to install ESXi 4.x onto a USB or SD flash storage device directly attached to the server.
This article provides instructions on for installing ESXi onto a USB drive or SD flash card.
Note: In the event that your original SD flash card or USB drive fails, these steps can be used in a disaster recovery capacity to get a fully functional ESXi host running as quickly as possible on a replacement USB drive or SD flash card.