I have run across this issue several times when running ESXi on Cisco UCS C-Series servers. ESXi is installed on SD cards connected to the Cisco FlexFlash controller. A fault occurs on the Flex Flash controller and the SD cards become inaccessible.
The host continues to run and there is no impact to virtual machines running on the host. There is a great FAQ about running ESXi from a SD or USB driver here that gives some more insight into why when the SD/USB device becomes inaccessible the host continues to run. The important thing to note here is that “once ESXi is loaded, it runs from memory and there is very little ongoing I/O to the boot device.” Once ESXi has booted the USB drive or SD card can be removed and the host will continue to run, of course configuration changes will not be saved and you will receive the bootbank error/warning.
An event is logged “Issue detected on esxi01 in Datacenter: Bootbank cannot be found at path ‘/bootbank'”
The host also displays a warning on the Summary page in the vSphere Client stating there is no persistent storage and configuration changes made to the host will not be saved.
This is a known bug with firmware versions earlier than 1.5(3d) on Cisco UCS C-Series servers and this article provides information on correcting the fault with the FlexFlash Controller without impacting production virtual machines.
Upgrading the CIMC firmware to 1.5(3d) or newer seems to fix the issue.