Bios/UEFI outputs are not displayed with external monitor? This is how it works.
If you connect an external monitor to a laptop, the outputs from the BIOS/UEFI may no longer be displayed. This makes it difficult, for example, to call up the boot menu.
In such cases, it is often worth taking a look at the BIOS/UEFI itself. They often offer a menu item that determines whether and how the boot outputs are displayed. On a Thinkpad, we find the settings in the configuration. Here we can determine whether the output is shown on the “Thinkpad LCD” or the external display.