EFI system partition and system partition

In short: The EFI system partition (ESP) is a small FAT32 partition in which the UEFI firmware finds the startup files (bootloader). The system partition, by contrast, in Windows terms is the partition with the boot files, and these terms are often confused.

In more detail: The firmware can read only FAT file systems. That is why the bootloader sits on a FAT32 partition of its own. From there the chain starts that finally accesses the actual Windows partition.

In Depth

Name	Content	File system
EFI system partition (ESP)	Bootloader (\EFI\Microsoft\Boot\bootmgfw.efi), BCD, boot drivers	FAT32
System partition (Windows term)	on BIOS systems the part with the boot manager (bootmgr), on UEFI the ESP	NTFS / FAT32
Boot partition	Windows itself (C:\Windows)	NTFS

Note: At Microsoft the “system partition” is the one with the startup files, the “boot partition” the one with the operating system. This sounds reversed but is defined that way.

Directory of the ESP (Windows):

\EFI\Microsoft\Boot\bootmgfw.efi   boot manager
\EFI\Microsoft\Boot\BCD            boot configuration data
\EFI\Boot\bootx64.efi               default load path of the firmware

• Linux uses the same ESP: There lie \EFI\ubuntu\grubx64.efi or systemd-boot. Dual-boot systems share one ESP.
• Type GUID: C12A7328-F81F-11D2-BA4B-00A0C93EC93B. This is how the firmware recognizes it.
• Repair: If the ESP is deleted or damaged, the computer does not start (“No bootable device”). In the Windows recovery environment: mount the ESP with diskpart, then bcdboot C:\Windows /s S: /f UEFI.
• Size: 100 MB suffice for Windows. Several Linux kernels or firmware updates need 500 MB to 1 GB.
• Security: Whoever can change the ESP can swap the bootloader. Secure Boot checks the signature against that.

See also: Bootloader, Boot files, GPT (GUID Partition Table)