Symptoms: BIOS Settings Reverted After CMOS Battery Replacement

• A Dell PowerEdge R640 had its CMOS battery replaced due to a "CMOS battery low" warning in iDRAC.
• After battery replacement and power-on, the server fails to boot to the OS.
• The server stops at the BIOS POST screen with "No bootable device found."
• Entering BIOS setup reveals all settings have reverted to factory defaults.
• Boot mode is set to Legacy BIOS instead of UEFI (the OS was installed in UEFI mode).
• Intel VT-x and VT-d (hardware virtualization) are disabled; this is a KVM hypervisor host.
• NUMA is set to "Node Interleaving Enabled" instead of the required "Disabled" (NUMA-aware).
• The server hosts 12 KVM virtual machines that are currently down.
• The expected BIOS settings were documented in a configuration management database but never exported as a BIOS profile.
• The technician who replaced the battery did not record the original BIOS settings beforehand.