Yesterday I set up my new PC. I installed Windows 10 via a USB boot drive. My PC has an SSD which I wanted to use for the OS and important applications and then I‘ve got an HDD with 4TB for mass storage. I installed Windows 10 on my SSD. When I then after the install was completed tried to format my HDD (using Windows 10 built in tools) I saw that Windows for whatever reason did two stupid things. Firstly it automatically formatted my 4TB HDD with MBR instead of GUID which is why only 2TB of the 4TB can be used at the moment (even though Windows support says Windows automatically chooses a suitable partition scheme). That alone would not be a problem as I could easily reformat the drive. The problem is that Windows 10 for some stupid reason I cannot quite fathom decided to use ~500MB of the 4TB HDD and use it as some kind of security/recovery/etc partition... I do not know why Windows even did that because I told it to use the SSD for the OS. Now here is my problem: because Windows 10 allocated part of my HDD for itself I cannot reformat the drive or do anything else with it. I would really appreciate it if you know how to reformat my HDD (using GUID) so I can use all the 4TB of the drive. I would also like to know how to tell Windows to stop using the HDD for OS purposes (just the SSD). Is this by any chance possible without reinstalling the OS? Thank you for your time and answers! PS: Some further information: when I tried to boot into Windows while the HDD was physically disconnected Windows 10 did not boot up. My BIOS is in UEFI mode.
The reason it may have automatically chosen MBR instead of GPT is because maybe UEFI was disabled in your "BIOS menu." I've encountered multiple computers where by default in the BIOS, it's set to legacy boot, with UEFI disabled. GPT partitioning only works when UEFI is enabled in the BIOS menu. If it's disabled, Windows will default to MBR. To be honest with you, I would start with a fresh install of Windows. First, make sure any files are backed up that you need. Second, enable UEFI and disable legacy boot in the BIOS. Then start the installation process over again by installing Windows to the SSD. Then once Windows is up and running, plug in your HDD and format that.