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I found EFI partiton on my main drive.

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Probably, the presense of this partition and it's contents prevent my computer from booting Linux, resided in another partition.

If I delete this partition, will my computer turn to norlmal legacy non-UEFI boot, or it will be bricked (not boot)?

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Without that partition your computer will almost certainly not boot. The UEFI firmware on your PC looks for a partition containing the system bootloader to execute and having deleted that partition it will simply be waiting for you to insert some kind of bootable CD-ROM or DVD that can install a bootloader and potentially an operating system.

It will not simply "fall back" to a "legacy" (BIOS based) boot, because it is not that simple.

  • UEFI expects a GPT partition format on the disk, the Windows BIOS bootloader expects an MBR partition scheme.
  • as you (probably) do not have an MBR partitioned disk then you will likely not have set up a boot sector on the disk pointing to a valid bootloader that the BIOS will be able to execute.
  • you need to set "legacy" boot mode in the BIOS.

Windows will also refuse to install to an MBR partitioned disk while the system firmware is set to UEFI mode, it will also refuse to install to a GPT disk on a system with a legacy BIOS setup. Microsoft has simply decided not to support these cases in their bootloaders.

Windows support for disk partitions can be found at Windows support for hard disks that are larger than 2 TB

System        BIOS + MBR   UEFI + GPT                  BIOS + GPT                   UEFI + MBR
Windows 7     Supported    Supported; (64-bit only)   Boot volume not supported     Boot volume not supported
Windows Vist  Supported    Supported; (64-bit only)   Boot volume not supported     Boot volume not supported
Windows XP    Supported    Not supported              Boot volume not supported     Boot volume not supported
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  • Well UEFI Windows Boot Manager does work on a disk with MBR partition table. As long as you can allocate space for a FAT32 ESP, you can use bcdboot to install a set to it and your Windows installation will be able to boot in UEFI mode. MBR boot code of Windows does not know how to handle a GPT disk though, for its job is essentially identifying the active partition on an MBR partition table and execute the boot code on the boot sector of that (which loads bootmgr).
    – Tom Yan
    Aug 11, 2017 at 23:04
  • BIOS understands neither MBR nor GPT. It's the boot loader that (may) interpret the partition table. Fundamentally, the issue in the question is one of boot loader support -- deleting the ESP deletes the boot loader(s), as you've said; and you'd need to replace them with something else, which would be a non-trivial task.
    – Rod Smith
    Aug 14, 2017 at 13:52
  • @RodSmith I've edited and hopefully gotten rid of the BIOS understanding MBR part. I probably should have added a note on why having that partition suggests that the system is indeed UEFI.
    – Mokubai
    Aug 14, 2017 at 14:33
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Probably, the presense of this partition and it's contents prevent my computer from booting Linux, resided in another partition.

This is incorrect. Most Linux distributions boot fine in EFI/UEFI mode, using an EFI boot loader stored on an EFI System Partition (ESP). You haven't provided much in the way of detail about your Linux boot problem, so I can't comment on it in detail, but the mere presence of an ESP is NOT the source of that problem. You may have installed in the wrong mode (BIOS/CSM/legacy rather than EFI/UEFI), a bug or user error may have caused the boot loader to not be properly installed, there might be a hardware incompatibility (especially if you've got a very new computer), etc. Deleting the ESP will just create new problems, so DON'T DO IT!

I recommend you post a new question that provides details about your problem with Linux. Include the name and version number of the Linux distribution you're trying to install and describe the symptoms you're seeing (for instance, is the installation medium not booting, is the installation failing with an error message, does the installation seem to complete but then boot straight into Windows, etc.).

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