This is purely a copypasta error fix, which was causing MountMgrNextDriveLetterWorker
to fail as no drive letters were enumerated previously.
With that set of patches, MountMgr now properly assigns drive letters to
new devices!
Being TRUE doesn't mean the device is GPT and has a drive letter. It just
means that it's not a GPT device with GPT_BASIC_DATA_ATTRIBUTE_NO_DRIVE_LETTER
attribute. In short, if TRUE, it means that the device can receive a drive
letter mount point.
This fixes MountMgrNextDriveLetterWorker bailing out for any attempt to
assign a drive letter to a device.