If breaking a mirror or adding hardware has changed your System/Boot drive letter, and you can not change it back in Disk Management, use the following procedure while logged on as an Administrator:
NOTE: DriveWrong refers to the current incorrect drive letter. DriveNew referes to the original drive letter.
1. Use Regedt32 to grant Administrators Full Control on HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices, using Security / Permissions. Exit Regedt32.
2. Using Regedit, locate the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices\DosDevices\DriveNew: value name.
3. Right-click and rename it to an unused letter, such as \DosDevices\Z:. This makes the DriveNew letter available for step 4.
4. Right-click the \DosDevices\DriveWrong: value name and rename it to \DosDevices\DriveNew:.
5. Select the \DosDevices\Z: value name and rename it to \DosDevices\DriveWrong:.
6. Exit Regedit and start Regedt32. Change the Administrator permissions on HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices to Read.
7. Check / change the PagingFiles drive letters.
8. Restart your computer.