You can back up and restore the ESXi configurations as below
Question is, even if you have a spare drive in the RAID configuration - won't you need to first install ESX on the second drive as well? There are other issues as well to take note of! Let's say you have got ESX installed on two HDD's and HDD1 is the active HDD. If you upgrade the ESXi installed on the first partition, and then you take a back up and keep it for later use, later the HDD1 fails and you restore the settings to ESX installed on HDD2, the versions of ESX would differ causing boot to fail
