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I am trying to install Windows 10, on an Asus laptop that already had a previous Windows installation. The disk has two partitions, one for the OS, this one was already deleted, and the other holds data I want to save. The installation media is a USB stick created with Microsoft Media Creation Tool. The installation fails to start

Windows can not be installed to this disk. The selected disk has an MBR partition table. On EFI systems Windows can only be installed to GPT disk.

I was advised that it may be that it is because the installation USB is loaded in UEFI mode. I Checked the BIOS, and the USB is listed there with UEFI prefix.

There is one BIOS setting.

Legacy USB Support

set to Enabled, but still it looks like the USB is loaded UEFI.

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  • If there's nothing else on the disk you want to preserve, you can do as Ramhound suggests: Wipe the disk clean so that Windows can create GPT partitions. If you want to dual-boot with an existing OS, you'll need to boot the installer in BIOS/CSM/legacy mode. The details of how to do this vary from one computer to another, but it's also necessary for the installation medium to have a BIOS-mode boot loader installed.
    – Rod Smith
    Commented May 22, 2017 at 2:29
  • You are trying to install Win10 on what? From which media? How does USB enter this picture?
    – Ale..chenski
    Commented May 22, 2017 at 6:25
  • @Ramhound I revised the original post, to make a bit clearer. I cannot reformat the disk as I have the 2nd partition I want to keep. I dont know about what you say regarding Legacy mode, as I was told differently, and If I understand correctly what Rod is saying, he agrees with that.
    – OJNSim
    Commented May 22, 2017 at 7:23
  • You must make sure the CSM is enabled. It cannot be enabled at the same time as Secure Boot, so the option may be hidden. It’s usually in the “Boot” section of the firmware setup.
    – Daniel B
    Commented May 22, 2017 at 7:24
  • @RodSmith I revised the question to be clearer...what you are suggesting is what I am trying to do. I want to boot the installer in Legacy mode, and it seems that the bios enables that, but it still booting in UEFI. so I guess the question How to make the stick to be installed with BIOS-mode boot loader, while it looks like the Microsoft tool does do it. Isn't it?
    – OJNSim
    Commented May 22, 2017 at 7:25

2 Answers 2

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You have two choices:

  • Install in EFI/UEFI mode -- The installer booted in this mode but refused to install because the disk was an MBR disk rather than the GPT disk that Windows requires for an EFI-mode installation. Thus, to install in EFI mode, you must convert the disk to GPT. Several third-party disk utilities, including my own GPT fdisk (gdisk), can do this without damaging the partition you want to preserve. (Note, however, that no partitioning operation is 100% risk-free. Thus, I recommend making backups. I make that same recommendation because of the risks involved in installing a new OS, even without a partition table conversion.)
  • Install in BIOS/CSM/legacy mode -- If you can get the installation medium to boot in BIOS mode, you can install to the disk in its current MBR configuration. The trick is to get the installer to boot in BIOS mode. The details of how to do this vary from one computer to another. As a general rule, you must enable an option in the firmware setup tool called "CSM support," "legacy boot support," or something similar. (The "legacy USB support" option you found is not the right option -- that option has to do with how USB keyboards and mice are treated, not to BIOS- vs. EFI-mode booting.) You may also need to select the installation medium via an option in the boot manager that names the device without the string "UEFI" appearing.

As a general rule, I recommend doing EFI-mode installations whenever possible. See this question and my answer to it, along with this web page of mine on the CSM, for details of why I recommend EFI-mode booting. In brief, EFI mode offers some modest advantages over BIOS mode, such as (usually) a slightly faster boot time and (on newer computers) Secure Boot. EFI is also the native boot mode; booting in BIOS mode basically requires running a BIOS emulator (the CSM) on the firmware, which complicates the boot path. OTOH, some early EFI implementations (from ~4 or more years ago, mostly) were flaky, and there is that modest risk involved with the MBR-to-GPT conversion. In some cases, these issues may outweigh EFI's advantages, which are far from overwhelming for most users.

EDIT:

The output you note (MBR: MBR only along with GPT: Present) indicates your partition table is in an inconsistent state -- the disk has what looks like a valid MBR, but there are also traces of GPT data. The usual cause is that the disk had been used with GPT, but at some point a GPT-unaware tool was used to create a fresh MBR partition table on it. If this is the case for you, you should pick the 1 - MBR option in rEFInd to have it convert the MBR data to GPT form. If you're uncertain, though, you can try to load the GPT data to see what it looks like, but do not save it back out with the w option -- at least, not unless you become convinced that the GPT data are more accurate than the MBR data.

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  • This makes the picture clearer for me. I backed up the data, and I want to Perform MBR to GPT conversion. I read the manual, and I am not sure what to do. Partition table scan (MBR: MBR Only,BSD: Not Found,APM: Not Found,GPT: Present). Now the menu (1 - MBR, 2 - GPT,3 - Create blank GPT). What do I need to choose?
    – OJNSim
    Commented May 22, 2017 at 20:41
  • Please see my edit, above.
    – Rod Smith
    Commented May 23, 2017 at 1:34
  • I do not understand what "GPT data" nor "MBR Data" mean, and what should I check exactly. Also, assuming one is more accurate, whats the next action should be. In addition, as I backed up the data, maybe I should wipe everything, and start from fresh. Can I do that? As I am walking in an unfamiliar territory here, I need ab exact instructions as for what to do.
    – OJNSim
    Commented May 23, 2017 at 7:14
  • MBR and GPT are two types of partition tables. See the linked-to Wikipedia entries. Windows ties these to boot types: MBR to BIOS and GPT to EFI. As I say, it looks like the disk used to be GPT, but you installed Windows in BIOS mode, which put a fresh MBR on it while leaving most of the GPT data intact and ripe to create confusion. I can't be 100% positive of this, though; there could be something more exotic going on.
    – Rod Smith
    Commented May 23, 2017 at 12:52
  • I understand that. What I Don't understand, is what do you mean by "MBR data" vs. GPT data. Is it the files you see when loading in bios mode vs. GPT mode? And actually now, it doesn't really matter anymore, as I managed to back up what I needed, something I had trouble with at the beginning. I want to wipe out everything, and reformat the disk to support the GPT mode installation. The question is specifically what option to choose in your tool to do that. Is it the 3rd option - Create blank GPT?
    – OJNSim
    Commented May 23, 2017 at 19:38
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I have the same issue once. If you have only one "Legacy USB Support" option it's not normally. Try to update you BIOS/UEFI and some more option will be avaliable after. "OS Type", "Boot Mode Selection", "Storage Boot Option Control" where you can switch UEFI/Legacy mode.

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  • First, it's not very clear what you're saying. Second, there is already a very detailed answer posted a long time ago. There already being answers doesn't mean you cannot post your own, but it does mean you'll have to have something valuable to add in order to make your answer similarly good.
    – music2myear
    Commented Jun 14, 2019 at 18:04

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