MOUNTMGR_DRIVE_LETTER_INFORMATION - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTIOAPI_H

// Output structure for IOCTL_MOUNTMGR_NEXT_DRIVE_LETTER.
typedef struct _MOUNTMGR_DRIVE_LETTER_INFORMATION
{
    BOOLEAN DriveLetterWasAssigned;
    UCHAR CurrentDriveLetter;
} MOUNTMGR_DRIVE_LETTER_INFORMATION, *PMOUNTMGR_DRIVE_LETTER_INFORMATION;

#endif

View code on GitHub

This structure is documented in Windows Driver Kit.