Error 0x80310051

Value: -2144272303 | 0x80310051 | 2150694993

What does it mean ?

FVE_E_RAW_BLOCKED  
BitLocker Drive Encryption cannot enter raw access mode for this drive because the drive is currently in use.
Value: 81 | 0x0051 | 0b0000000001010001

Where does it come from ?

FACILITY_FVE  
Full volume encryption
Value: 49 | 0x031 | 0b00110001

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