#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

// mountmgr.h

typedef struct _MOUNTMGR_DRIVE_LETTER_INFORMATION {
  BOOLEAN DriveLetterWasAssigned;
  UCHAR   CurrentDriveLetter;
} MOUNTMGR_DRIVE_LETTER_INFORMATION, *PMOUNTMGR_DRIVE_LETTER_INFORMATION;
View the official Windows Driver Kit DDI reference

NtDoc

This structure is documented in Windows Driver Kit.

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Windows Driver Kit DDI reference (ns-mountmgr-_mountmgr_drive_letter_information)

Description

The MOUNTMGR_DRIVE_LETTER_INFORMATION structure is used by the mount manager to furnish a drive letter to a client that has requested a driver letter by means of an IOCTL_MOUNTMGR_NEXT_DRIVE_LETTER request.

Members

`DriveLetterWasAssigned`

Indicates when TRUE that member CurrentDriveLetter contains a drive letter. When FALSE, a driver letter was not assigned to the device.

`CurrentDriveLetter`

Contains either an existing or a newly assigned drive letter in the form of a single ASCII character (for example, "D") if DriveLetterWasAssigned is TRUE.

Remarks

For more information, see Supporting Mount Manager Requests in a Storage Class Driver.

MOUNTMGR_DRIVE_LETTER_INFORMATION - NtDoc