I recently installed Linux in an UEFI laptop and installed the GRUB loader wrong (MBR based). After this the system did not start in any OS. The boot device selector still listed the UEFI devices, but whatever was selected it did not boot. I tried recovery with Windows tools (fixMbr) but nothing helped out. Finally I solved by resetting the MBR (from within Linux recovery) and reset the protective flag. After this I could install GRUB "correct" registered as UEFI and I could select whatever UEFI device was available. What I am really interested in is why this happened and why the UEFI system denied to boot with this MBR not being correct. There must be a mechanism which compares the UEFI signature against the MBR. Is my assumption correct or might anything else explain this phenomenon?

  • Hi downvoters, if you think you need to downvote this question please consider to explain, maybe I can improve. – Thomas Oct 9 '17 at 12:23
  • UEFI is not designed to boot an MBR-partitioned system It was designed to work with GPT-partitioned disks. – fpmurphy Oct 9 '17 at 14:37

Because BIOS use MBR partition style instead of GUID partition table that UEFI use.

See Working Around MBR's Limitations and What's a GPT?.

Pay attention to article Legacy BIOS Issues with GPT:

Unfortunately, in practice, there are occasional BIOS/GPT quirks or even downright incompatibilities. Furthermore, many modern computers with EFIs provide a Compatibility Support Module (CSM), which enables them to boot using BIOS-mode boot loaders. The trouble is that the firmware may use the partition table type as a cue for what type of boot mode to use, thus causing problems when trying to boot in BIOS mode from a GPT disk. In either case, a computer with a GPT disk may not boot ...

  • The BIOS was not changed, no Legacy or CSM enabled, so you say it switched in presence of a MBR? – Thomas Oct 9 '17 at 12:24
  • I mean that BIOS-based computer use MBR partition style. But modern UEFI computers (such as your laptop) use GPT partition style. Some problems exist with mbr and gpt boot compatibility – Egor Vasilyev Oct 9 '17 at 12:31
  • many UEFI systems will boot just fine from an MBR-partitioned disk, if that disk contains a correctly-prepared EFI System Partition (ESP). UEFI's standard booting looks for a fat32 partition, rather than a specific position on the disk (like how BIOS boots). – quixotic Oct 9 '17 at 13:30

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