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On a fresh (custom) installation of Windows 7, where Windows 7 is installed onto a new hard disk with unallocated disk space (no partition or volume been defined yet), or when user attempts to create a new partition out of empty drive, the Windows 7 installer will create an additional partition with the size of 100.00 MB, and mark as System Reserved.
The 100MB volume is labeled as System Reserved with NTFS file system, and System, Active, Primary partition attribute with no drive letter in Disk Management. The 100MB system reserved partition is only available for Windows 7 Ultimate, Windows 7 Business and Windows 7 Enterprise editions. The 100 MB system partition is used primarily as BitLocker partition for BitLocker encryption. Additionally, it also holds the Windows Recovery Environment (WinRE) and boot files with boot manager for booting up the computer for troubleshooting when there is no Windows 7 installation DVD disc on hand.