Hypothetical. Say, for example, you’ve got a ZFS mirror on
disk1, from which your Nexenta system boots. A year or two later,
disk0 crashes, and starts spewing messages such as the following:
ahci: [ID 860969 kern.warning] WARNING: ahci0: ahci_port_reset port 0 the device hardware has been initialized and the power-up diagnostics failed
Shortly thereafter there’s a storm and the power goes out for several hours and drains your entire humongous UPS, and when the power finally comes back on, the system won’t boot — it stops at the
grub prompt; issuing a standard
boot command fails.