How it works
Legacy BIOS and UEFI are firmware approaches used to initialise hardware and start an operating system. UEFI provides a newer framework with features that can include Secure Boot and different boot-management methods. Some systems also expose a compatibility mode for older installations. The firmware mode, disk partition layout and installed boot files must agree; selecting a different mode is not merely changing the appearance of the setup screen.
For example, an existing system installed for legacy boot may stop starting after a switch to UEFI-only mode even though its disk remains physically healthy. Windows UEFI installations commonly use GPT partitioning, while legacy arrangements use MBR. A supported conversion is a planned process with prerequisites and recovery preparation. Reformatting is a separate destructive action, not a required first attempt whenever a boot-mode mismatch appears.
Practical checks
Record the current firmware mode and partition style before altering settings. Back up data and preserve encryption recovery information. Check the operating system's and manufacturer's supported conversion or installation procedure for the exact hardware. Change only the documented settings and test startup. If a recent mode change caused the fault, restoring the previous known configuration may help diagnosis. Do not disable security features or erase the disk simply to bypass an unexplained boot error.