In ESXi 6.5, a new USB driver was introduced to replace the legacy linux based USB and XHCI drivers to be used for communication with SCSI based SD cards seen through onboard USB based controllers.
While the cause of the events may vary and depend on the HW layer, small disconnections from the SD card via the USB based interface while using the new vmkusb driver can result in an APD event that will most likely render the ESXi OS install location to drop. This causes the ESXi system to then go into read only mode, however, it can still run and operate under most normal conditions within vCenter and still service virtual machine operations.
Further, the ESXi OS may recover from the APD event once the SD card reconnects if the SD card is not configured to have an active core dump partition on it. This, however, this is not a suggested workaround.
This issue is not seen to re-occur inside the Linux based legacy USB driver. This is believed to be due to the maturity of the legacy code and that the resiliency on retrying loss connections to the USB device is more flexible, thus it recovers without any subsequent events or impact.