Many ESXi installations are running on SD Cards or flash drives. In my opinion, it's a good practice. The hypervisor itself requires about 150MB, and the full installation on a SD Card is less than 1GB, without diagnostic partitions. VMware recommends using a 4GB or larger USB/SD device. When you want to install ESXi and you don't use auto-deploy, install servers or other automation tools you typically have to mount an ESXi ISO file to your server management system (iLO, iDRAC,...) or work with a physical installation media. This is somewhat slow and uncomfortable, but there is a little trick to make the installation faster.
You can prepare the USB/SD device with the ESXi installer, plug it into your server and install it to the device itself by overwriting the installer. You can also use customized installers when your hardware requires special drivers.Read More »Pre-installed ESXi 6.0 on SD Cards or Flash Drives