This behavior is by design. Backup Exec recognizes FSRM volume as critical resource. Any exclusions from a drive determined to be critical by the program will cause the SDR indicator and functionality to be turned off.

•  Stop FSRM service

      OR

• Exclude FSRM volume from a Simplified Disaster Recovery
  
  Note: 
  Volumes that were managed by FSRM need to exist at the time of recovery. If the volumes don't exist at the time of recovery, please add additional disks that were managed by FSRM. Added disks should not be of the same size.
  
  Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.
  
  1)  Launch the registry editor on the computer being backed up
  
  2) Navigate to the HKLM\SOFTWARE\Symantec\Backup Exec For Windows\Backup Exec\Engine\Simplified System Protection
  
  3) Create a new key called "User-Defined Exclusion Resources" under the key “Simplified System Protection”
  
  4) Create a new key called FSRM volume name e.g. “E:” under the key "User-Defined Exclusion Resources"
  
  5) Check SDR backup can run without FSRM volume