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I was on Windows 7, with 4 partitions. I decided to do a complete clean up and upgrade to Windows 10.

So, while doing that, I selected the option, (I don't remember it exactly, but) clean up everything and upgrade.

Windows 10 opened and the partitions were still there with my data. I tried resetting, but it said "Cannot be reset. No changes were made" (Something very near that.)

I went on Microsoft's website and downloaded and created my USB (FAT32) as the boot media.

I booted from the USB via legacy boot and got to the installation screen. Deleted my partitions, except for the main one then when I tried to install Windows 10 on my main partition, it said, "Windows can't be installed on MBR. Convert to GPT." (My partitions are NTFS) Now, I have no idea what to do. I saw a solution that uses CMD,

diskpart
list disk
select disk <disk number>
clean
convert gpt
exit

But, I'm afraid if I do that, it'll clean up my main partition then if it doesn't work, I'LL BE LEFT WITH NOTHING. Please advise.

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    You probably started the installation in UEFI mode. UEFI requires a GPT partition table. If you want to convert the MBR (Master Boot Record) table to a GPT table, you have to completely clean/format/wipe your hard drive. There are tools that claim to be able to convert MBR to GPT, but they only work on non-boot, external drives that are not running (I tried to convert my HDD some time ago, but then simply stuck to MBR because I don't need more than 4 partitions).
    – GiantTree
    Commented Nov 25, 2015 at 16:26

3 Answers 3

Reset to default
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Yes don't do that! Clean will quite literally clean/format the HDD/SSD.

There isn't really a way to convert from MBR to GPT without destroying the disks contents though as far as I'm aware (unless times have changed?).

The only advise I could give is back everything up and flatten the HDD and install Windows 10 fresh.

... Though Windows 10 /shouldn't/ require GPT, are you sure you didn't boot it up as a UEFI install?

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    Yo, your "are you sure you didn't boot it up as a UEFI install?" made me check and I did. You were right. I booted up as Legacy and it worked. But then, when I finally selected the partition and proceeded, it started saying, "cannot proceed. deployment problem" or something, and asked me restart the installer. I did that a couple of times, then instead used the repair option, which at first seemed to work, but stopped in middle xD Again, I tried the normal install, and it worked by itself... So, thanks because without you I wouldn't have gotten anywhere :D
    – user3192201
    Commented Nov 25, 2015 at 21:06
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For people configuring their PCs for the first time all by themselves, an error message like “Windows cannot be installed on this Disk” can be a big time problem. One, the message explains little about its cause and second, most of us don’t really have a lot of experience in setting up systems. The problem, however, is not as elephantine as it seems. It is simply a compatibility issue. This problem generally occurs when you try to install windows on a UEFI-based system while your hard drive partition scheme does not support UEFI.

The agreement between the firmware and the hard disk partition scheme is essential for locating the boot loader for the Operating System and hence, is the first step in the installation process of windows on a computer system. By default, recent hardware boot using UEFI mode. Thus, when one tries to use an old hard disk with an updated UEFI-based system, the message “Windows Cannot be Installed On this device” pops up. This is because old hard drives maintain partitions using MBR scheme which is not supported in UEFI-based systems. Therefore, for windows installation it is important to convert either MBR partition scheme to GPT partition scheme or configure the system to boot with the legacy BIOS-compatibility mode.

To solve this problem without data loss, you need to convert MBR to GPT disk with third party software or enable Legacy boot mode in BIOS.

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Change your bootable option, My DVD drive in bootable priority in setup screen has two option, One with EFI prefix and other without it, I used DVD drive without EFI prefix and problem was solved.

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