But some users are unable to switch back to the Legacy BIOS mode after enabling the UEFI BIOS mode. When they tried to do so, they found the Legacy Boot is greyed out in BIOS.

To enable or disable Fast Boot in UEFI/BIOS in Windows 11/10, do the following: Note: Depending on the PC or motherboard manufacturer, the steps, available Fast Boot settings, and interface may vary.

Something to keep in mind as you apply this technique. To add the legacy Advanced Boot Options to either of the current versions of Windows, open an administrative level command line prompt.