Under certain conditions, you may be unable to boot Windows Small Business Server (SBS) 2011 and may receive the following error:
BOOTMGR is missing. Press Ctrl+Alt+Del to restart.
This issue can occur if the System Reserved Partition is not marked as Active.
To resolve this issue, follow these steps:
1.) Boot your server from the installation disk (DVD 1 of SBS 2011 media)
2.) Click Next and then click 'Repair your computer'
3.) Select the first radio button, and then click Next.
4.) Click 'Command Prompt'
5.) Type Diskpart and press Enter.
6.) Type List Volume and press Enter. You would see a list of all the volumes on your system.
7.) The System Reserved partition is mostly around 100 MB in size. In the below example its Voulme 1 and is represented by the drive letter D.
8.) Type Select Volume 1 and press Enter.
9.) Type Active and press Enter. You would see a message 'DiskPart marked the current partition as active'.
10.) Type List Volume and press Enter. You would see an asterisk (*) against the volume that has the current foucs, in this case, its System Reserved partition.
11.) To verify the System Partition is now marked as Active, type List Disk and press Enter.
12.) Typically Disk 0 would be where your Windows is installed. Type Select Disk 0 and press Enter. You would see a message 'Disk 0 is now the selected disk'.
13.) Type List Partition and press Enter.
14.) Type Select Partition 1 and press Enter. You would see a message 'Partition 1 is now the selected partition'.
15.) Type Detail Partition and press Enter. As you can see the below example, the System Partition (which is Partition 1 on Disk 0) is now marked as Active.
11.) Exit DiskPart and reboot the server normally.
With the onset of Windows 7 / Windows Server 2008 R2, windows create additional partitions called System Reserved Partition. This partition is used by Windows for tools such as BitLocker Drive Encryption, Windows Recovery Environment (Windows RE) and to manage and load other partitions. The System Partition MUST be configured as the active partition for Windows to boot.