Looking at the preceding partition list, you may notice that partitions 5 and 6 are named primary boot bank and alternate boot bank. These partitions are a failsafe. The ESXi system has two independent banks of memory, each of which stores a full system image. When a fresh ESXi installation is performed, partition 6 is empty.

During the system upgrade, the new version is loaded into the inactive bank of memory and the updated bank is set to be used when the ESXi reboots. If the boot process fails for any reason, the system automatically boots from the previously used bank of memory. You can also manually choose which image to use for that boot at boot time.