I've usually formatted FAT32 drives MBR and copied W10 iso files over to USB and booted the installer in UEFI no problem. This time I did NTFS GPT and noticed two things: - It seemingly boots faster; had a hard time getting <4s Task Manager times post-tweaks but had 3s early before updates even - Device IDs for Intel DPTF and a I2C touchpad changed slightly (no idea what that's about; I disable em in registry and out of dozens of times, the IDs changed only this time) I was thinking something like MBR hinting that Windows should install CSM/Legacy support whereas GPT implies modern/full UEFI no CSM? Rufus's drop-down for GPT/MBR sort-of implies a difference (MBR mentions CSM, but if no-diff then why not always do MBR since UEFI can see it regardless?)
BIOS/Legacy and UEFI are firmware types. MBR and GPT are partition schemes. Both are completely independent from each other, except for MS, MS Setup dictates BIOS=MBR and UEFI=GPT. CSM is only for compatibility with older OS like Windows 7 which does not support GOP and needs INT10 BIOS emulation for boot graphics. Many modern UEFI 3+ systems do not have it anymore.
Does Windows add CSM support on UEFI if it's required? Rufus implies a difference: GPT: MBR: With GPT it specifies non-CSM, but MBR has UEFI-CSM, implying trying to boot GPT media on UEFI that isn't pure-EFI (no GOP?) wouldn't work, but also implying that MBR media supports both non-pure EFI and pure-EFI (potentially adding extras for non-pure EFI support)? If a USB is FAT32/NTFS formatted single partition and a Windows ISO contents copied directly to it, I'm wondering if Windows installer does something different if the USB was MBR (implying it might be Legacy so install extras), vs GPT (must be modern/no need for legacy files/support)
No, the CSM is in the firmware (UEFI until class 2). Class 3 UEFI in modern machines does not have CSM, and you can not use Windows 7 in UEFI mode on these (unless you emulate it with e. g. UEFISeven). The target system setting in Rufus is for the way you will boot the USB drive on the target machine. Booting the USB in UEFI mode will only allow installing in UEFI mode, and Windows Setup forces GPT partition style in this mode. Booting the USB in legacy mode will use that mode for Windows Setup, too, and forces MBR partition style for the installation drive (secondary drives can be created in GPT scheme later). Pressing the ALT+E cheat key in Rufus allows you to create a universal drive that can boot both modes (needs to be selected in the boot manager of your firmware). Note that, from my experience, modern Setup in Windows 11 might not be able to correctly install legacy/MBR in all cases; for me, it did not copy the legacy boot loader to the drive and the system failed to boot after the first restart. Previous version of Setup does not have that problem. The partition style is for the USB media itself, and UEFI without CSM forces GPT, as MBR makes no sense here. Legacy BIOS and UEFI class 2 with CMS can boot MBR media (UEFI class 2 can boot both). From what I know, simply copying the ISO contents to the target USB media works only for UEFI mode, as the firmware can access the UEFI boot loader BOOTMGR.EFI directly. MBR requires the Master Boot Record and a boot sector being correctly written to the USB, as that MBR chainloads the boot sector, which chainloads the legacy Windows boot loader BOOTMGR.