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There are many questions I need someone can help me answer it. Let me explain first... Longtime ago, my laptop only have 1 HDD and 1 CD-Drive. I have been using around 3 years, and my hard disk is suddenly at "Bad" status. I ask for my friend to find and buy another HDD for me, but he told me using SSD is better for later.

I said Ok, and he took my Laptop and went to the shop; after that, my laptop started very quickly, and the system looked smoother. When I start gaming, somehow, my laptop heat as usual, forcing me to sleep mode on Windows. I thought it was a Windows bug, but I tried reinstalling again, and the same happened (even after updating the new driver).

Later, I want to expand my hard disk to get the Caddy Bay SSD SATA 3 9.5mm/12.7mm and get the new HDD. I bring it to the other shop computer and asked him to remove Cd-drive and plug HDD. After 2 hours of waiting.. he says it has been done. After 1 month, my laptop suddenly changed boot OS when I powered it up. I did not recognize it until I spent 1 hour Googling it and found out.

Wait until the mainboard receives the signal from SSD for around 10 or 30 minutes, and the system shows the "Go to Bios" line; I go to the BIOS system, change the Boot mode from UEFI to Legacy mode, and it works!!! The windows start loading, and everything is normal. Now this error keeps happening after a few updates from Windows 10 or sometimes. I am very upset about this situation. I need to know:

When I switch from UEFI to Legacy mode, is it mean my mainboard supports boot UEFI right? If yes, please answer the second question.

I just google and checked; even my mainboard support boot UEFI but my hard disk does not correct the style GUID Partition Table (GPT), so I can't install Windows boot UEFI. Can I change the hard disk from Master Boot Record (MBR) to GUID Partition Table (GPT)? Will I lose data when I change from MBR to GPT?

If my mainboard support boot UEFI and my hard disk can change to GPT, is it possible to set the default boot UEFI for later and install new Windows as UEFI?

Could this error happen from battery CMOS? Maybe the battery is not good. I'm "dead some time," so my mainboard changed from Legacy to UEFI?

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  • "When I switch from UEFI to Legacy mode, is it mean my mainboard support boot UEFI right?" - No; If Windows is booting in Legacy Mode, it means that you have enabled CSM with the firmware of your motherboard. You provided zero information on your hardware. What problem are you trying to solve?
    – Ramhound
    Apr 28, 2023 at 9:35
  • "Can I change the hard disk ..." - Yes; You would have to get rid of your data partition to accomplish that with MBR2GPT. "Will I lose data when I change from MBR to GPT?" - If you use MBR2GPT, the conversion will be seamless. "Could this error happen from battery CMOS?" - No; If Windows were already installed, switching from UEFI to Legacy Mode would have caused your system not to boot; if that happened, all you had to do with disabling CSM to solve the problem. The fact Windows is configured to boot off of MBR and in Legacy Mode suggests CSM was always enabled.
    – Ramhound
    Apr 28, 2023 at 9:44

1 Answer 1

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I just google and check, even my mainboard support boot UEFI but my hard disk does not correct style GUID Partition Table (GPT) so I can't install Windows boot UEFI ? If so, can I change hard disk from Master Boot Record (MBR) to GUID Partition Table (GPT) ? Will I lose data when change from MBR to GPT ?

With recent Windows versions, it would be easiest to use the mbr2gpt tool that's included with the OS. It will perform both conversions at once, automatically.

If the tool does not work, you could use the gdisk tool (e.g. from a Linux live-USB, although I think it used to have a Windows .exe as well) to convert the partition table from MBR to GPT first without data loss, then reformat the "System Reserved" partition to FAT32 turning it into an "EFI System Partition", and finally use bcdboot from a Windows install USB to make the disk bootable again.

If my mainboard support boot UEFI and my hard disk can change to GPT, is it possible set default boot UEFI for later and install new Windows as UEFI ?

Yes, but keep in mind that this needs the installation medium to be UEFI compatible as well. That is, if you're using third-party tools (such as Rufus) to create the Windows install USB, make sure you've selected "UEFI" or "GPT/UEFI" format for the USB stick. (Windows' own "Media Creation Tool" always creates UEFI-compatible ones.)

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  • Appreciate your answer. But do you know the reason why my laptop sometimes change boot os from Legacy to UEFI ?
    – James Tôn
    Apr 28, 2023 at 8:28
  • @JamesTôn - We can't determine that based on the information you provided. Since you already have 4 partitions on a MBR disk, you won't be able to run MBR2GPT. MBR only supports 4 partitions.
    – Ramhound
    Apr 28, 2023 at 9:32

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