Writing files to external flash drive in Windows Universal

Question

I'm writing an application using Windows IoT on a Raspberry PI. I would like to write data to an external flash drive connected to one of the USB ports. I've found examples on how to write to the SD card in the PI, but the SD card won't be accessible in the final product.

I can get the root folder name of the flash drive, but when I try to write a file to it, I get an access denied message. If I switch to the SD card everything works fine.

Can anyone point me to an example that allows access to an external flash drive?

Answer 1

For security reasons, Universal Windows Applications can only have access to certain types of files on external drives,

• Music
• Picture
• Video

And you have to explicitly declare it in the Package.appxmanifest file.

• Music Library
• Picture Library
• Video Library

You might also want to check the Removable Storage capability as well.

I don't think you have access to a general file format except the above three types, otherwise you'll get an "Access is denied" exception.

Find more details in here.

Once you have your capabilities declared, you can get the root folder for your external storage device with the following code,

var removableDevices = KnownFolders.RemovableDevices;
var externalDrives = await removableDevices.GetFoldersAsync();
var drive0 = externalDrives[0];

Then you can use the Stream methods to write to a file, following the code samples in here.

If you want to write data to an generic file format, an workaround is to use an accessible file format(like jpg), and write your raw data to it. Below is some code sample that is verified on Raspberry Pi 2 Model B, with Windows IoT 14393, with an external USB drive connected to the USB port.

    private async void WriteData()
    {
        var removableDevices = KnownFolders.RemovableDevices;
        var externalDrives = await removableDevices.GetFoldersAsync();
        var drive0 = externalDrives[0];

        var testFolder = await drive0.CreateFolderAsync("Test");
        var testFile = await testFolder.CreateFileAsync("Test.jpg");

        var byteArray = new byte[] { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };
        using (var sourceStream = new MemoryStream(byteArray).AsRandomAccessStream())
        {
            using (var destinationStream = (await testFile.OpenAsync(FileAccessMode.ReadWrite)).GetOutputStreamAt(0))
            {
                await RandomAccessStream.CopyAndCloseAsync(sourceStream, destinationStream);
            }
        }
    }