WinAPI calls to access USB storage which has no drive letter?

Question

I\'ve noticed, that some USB storage devices don\'t register in Windows as regular drives, in that they don\'t even get assigned a drive letter. Therefore, I can\'t apparently a

Answer 1

If you're talking about a USB mass storage device without any assigned drive letter then it's just a volume without a mount point. You'll need to mount the volume before reading/writing it.

You can use the Volume Management Functions :

• FindFirstVolume and FindNextVolume to get the volumes GUID
• GetVolumePathNamesForVolumeName to know if the volume is mounted (and get the mount point)
• SetVolumeMountPoint to mount a volume

Here is a quickly-written example in C that list all the existing volumes, mount the unmounted ones and show some information about each volume :

char volumeID[256], volumePathName[256], volumeName[256], volumeFS[256];
char newMountPoint[4] = " :\\";
unsigned long volumeSerialNumber;
unsigned long size;
HANDLE handle = FindFirstVolume(volumeID, 256);
do {
    printf("Volume GUID = %s\n", volumeID);
    GetVolumePathNamesForVolumeName(volumeID, volumePathName, 256, &size);
    if(strlen(volumePathName) == 0) {
        printf("Not mounted\n");
        newMountPoint[0] = firstFreeLetter();
        if(SetVolumeMountPoint(newMountPoint, volumeID)) {
            GetVolumePathNamesForVolumeName(volumeID, volumePathName, 256, &size);
            printf("Now mounted on %s\n", volumePathName);
        }
    }
    else {
        printf("Mounted on %s\n", volumePathName);
    }
    GetVolumeInformation(volumePathName, volumeName, 256, &volumeSerialNumber,
                         NULL, NULL, volumeFS, 256);
    printf("Volume name = %s, FS = %s, serial = %lu\n\n",
           volumeName, volumeFS, volumeSerialNumber);

}while(FindNextVolume(handle, volumeID, 256));

FindVolumeClose(handle);

I deliberetely simplify this example, but a volume can have multiple mount points (volumePathName is actually a multi-string). It uses this function to get the first available letter (after 'C') to mount a drive :

char firstFreeLetter() {
    unsigned long freeLetters = GetLogicalDrives();
    if(freeLetters < 4) return 0;
    char letter = 'C';
    for(unsigned long i=4; (freeLetters & i) != 0; ++letter, i <<= 1);
    return letter;
}