When you disable Secure Boot, the Boot Mode switches to Legacy. Hit the pulldown on Legacy and set the Boot Mode to UEFI and leave Secure Boot Disabled. If you leave it in Legacy, you'll have to reinstall Windows.
You can run either:
- in legacy mode (which doesn't have a Secure Boot option);
- in UEFI mode with Secure Boot; or
- in UEFI mode w/o Secure Boot
Legacy mode uses the older MBR disk partitions. UEFI Mode uses the newer GPT disk partitions. Dell installed Windows in UEFI mode, when you boot in Legacy mode, Windows can't read the data in the GPT partitions.
It would be nice if when you switched the Secure Boot from Enabled to Disabled the Boot Mode stayed in UEFI and didn't switch to Legacy. But that's not the way the BIOS Setup is currently working.
