the only HDD that i have a 3TB Hitachi that I bought. Or this could be a limitation from the old MOBO. If so, how can I locate the missing 2TB that during windows Disk Management does not show or using minitool partition?
OP - you need to disable UEFI in the BIOS, convert your disk to MBR, install Win 7, activate using Daz loader, upgrade to Win 10, and confirm activation status. Now you can do a clean install of Win 10 if you like. If you want to use UEFI and GPT, you may now convert your disk to GPT again and reinstall Win 10. Good idea to delete an existing partitions and install Windows to unpartitioned space and let it make whatever partitions it likes, only choosing the size of the main partition so you can create additional partitions later.
Old MOBO that I using for a cheap computer for my sis. 780i SLi. It does not have an option within BIOS to DISABLE UEFI
even when i connect the HDD to my main PC it only shows 746GB. With Minitool Partition, if I change it to GPT it will show the unallocated amount, but once I merge, or change to MBR it will show again only 746GB. This is with my windows 10 PC. I cant get it to show me at least 2TB in MBR so I can install windows 7. Because as stated above, it keep giving me the above message. I have been using Gparted live boot but when I create a new table and I size the partition it automatically registers them as GPT
CLEAN the disk. This deletes ALL partitions. All the space should now show as unallocated in Disk Management. This PC>Manage>Disk Management.
As I was read your OP and some other posts, your Main problem is to get Windows 10 installed and auto-activated. I posted the required ways several times the How too.....! I'll do that now the last time and you may understand what I'm talking about: use any HDD connected in your computer, best as single HDD connected and install Windows 7 Ultimate to it use DAZ Loader to activate Windows 7 Ultimate us a Windows 10 Pro ISO, or Flash drive, or DVD (all Bootable) and from within the running Windows 7 Ultimate, start via Windows Explorer/File Manager the Upgrade process by executing the setup.exe file of Windows 10 Pro as asked for a Product Key 'check' that you didn't have one finish the installation check Activation: it should say Windows 10 Pro is activated using a Digital License Remove or format the used HDD with the activated Windows 10 Pro add all required/available/needed HDD's and or SSD's to the computer start a new clean install of Windows 10 Pro without using any Product Key end up with a clean installed Windows 10 Pro activated with a Digital License Got it? After Point 7 above, you could use any Boot/Partition Scheme for to install Windows 10 Pro. Keep in mind that the Digital License is bound to that Mainboard which you used for install Windows 7 Ultimate and upgraded that to Windows 10 Pro (point 1-6 above). Don't forget to be connected to the Internet while upgrade and /or installing Windows 10 because of the HWID, needed for the Digital License to build (Point 3) and Auto-Activate later on, need to check with MS Servers.