Dual Boot Failure

I have an old HP CTO with Win7-64. Runs just fine. I would like to do do a dual boot and add Win 10 Full Retail Version. I currently have 3 HD's installed. Drive 0 (2TB's) is the Win7 OS. Drive 1 is for backup data as is Drive 2. Drive 0 (C:) is listed as a GPT. I added another partition to Drive 0 in hopes that I could install Win10 to that partition. Didn't work. Error message was that Win10 could not be installed because the drive (0) was GPT.

I then attempted to install Win10 to Drive 2 which was listed as a MBR. I then got error message 0x80300024. A search yielded info that I should disconnect extra HD's and try a reinstall of Win10. Drive 0 (C:) has the Win7 OS. Obviously I can't disconnect this.

Question: Should I consider converting Drive 0 from GPT to MBR and would this require a complete reformat to do so? Is this doable without a reformat and is there a tool to do this? Can I change just the new partition to MBR without having to change the entire Drive?

Question: Any other solution to error message 0x80300024?

Thanks

The problem simply is that you booted the Windows 10 flash drive in legacy BIOS (CSM) mode. Boot the flash drive in UEFI mode and you can install it to the partition on the GPT drive.

NavyLCDR said:

The problem simply is that you booted the Windows 10 flash drive in legacy BIOS (CSM) mode. Boot the flash drive in UEFI mode and you can install it to the partition on the GPT drive.

The Win 10 is a full retail version on a DVD, not a flash drive. Should I copy/paste the DVD data to a flash drive and proceed with your suggestion?

As long as you have a key download the latest ISO free from Microsoft and let it create a USB as the DVD you have may not have it and it may be an out of date one so once you install you would get it all done again from update

Please try the following:

a) Insert the bootable Windows DVD into the optical drive
b) turn off the computer
c) physically remove the drives or detach the cables to drives other than the designated drive for a clean install
d) power on the computer
e) perform a clean install using the DVD
Clean Install Windows 10

Samuria said:

As long as you have a key download the latest ISO free from Microsoft and let it create a USB as the DVD you have may not have it and it may be an out of date one so once you install you would get it all done again from update

Yes, I have the key that is assigned to the DVD and I was able to get to the stage of the install where I was asked to which drive I wanted to install Win 10. That is when I ran into the error messages. The first one being told that my Drive 0 was in GPT format. The second one when I tried to install Win 10 to Drive 2 which is in MBR format. From your statement, I get the impression that the key on the DVD is usable with an ISO download. Is that correct?