You can take advantage of ManagementObjectSearcher using it to query the disk drives that are USB, then obtain the corresponding unit letter and return only the DriveInfo of which RootDirectory.Name is contained in the result set.

Using LINQ Query Expressions:

static IEnumerable<DriveInfo> GetUsbDevices()
{
    IEnumerable<string> usbDrivesLetters = from drive in new ManagementObjectSearcher("select * from Win32_DiskDrive WHERE InterfaceType='USB'").Get().Cast<ManagementObject>()
                                           from o in drive.GetRelated("Win32_DiskPartition").Cast<ManagementObject>()
                                           from i in o.GetRelated("Win32_LogicalDisk").Cast<ManagementObject>()
                                           select string.Format("{0}\\", i["Name"]);

    return from drive in DriveInfo.GetDrives()
           where drive.DriveType == DriveType.Removable && usbDrivesLetters.Contains(drive.RootDirectory.Name)
           select drive;
}

Using LINQ Extension Methods:

static IEnumerable<DriveInfo> GetUsbDevices()
{
    IEnumerable<string> usbDrivesLetters = new ManagementObjectSearcher("select * from Win32_DiskDrive WHERE InterfaceType='USB'").Get().Cast<ManagementObject>()
        .SelectMany(drive => drive.GetRelated("Win32_DiskPartition").Cast<ManagementObject>())
        .SelectMany(o => o.GetRelated("Win32_LogicalDisk").Cast<ManagementObject>())
        .Select(i => Convert.ToString(i["Name"]) + "\\");

    return DriveInfo.GetDrives().Where(drive => drive.DriveType == DriveType.Removable && usbDrivesLetters.Contains(drive.RootDirectory.Name));
}

Using foreach:

static IEnumerable<string> GetUsbDrivesLetters()
{                
    foreach (ManagementObject drive in new ManagementObjectSearcher("select * from Win32_DiskDrive WHERE InterfaceType='USB'").Get())
        foreach (ManagementObject o in drive.GetRelated("Win32_DiskPartition"))
            foreach (ManagementObject i in o.GetRelated("Win32_LogicalDisk"))
                yield return string.Format("{0}\\", i["Name"]);
}

static IEnumerable<DriveInfo> GetUsbDevices()
{
    IEnumerable<string> usbDrivesLetters = GetUsbDrivesLetters();
    foreach (DriveInfo drive in DriveInfo.GetDrives())
        if (drive.DriveType == DriveType.Removable && usbDrivesLetters.Contains(drive.RootDirectory.Name))
            yield return drive;
}

To use ManagementObject you need to add reference to System.Management

I haven't tested it well because now I don't have any SD card, but I hope it helps

I had to check for USB-Devices in an older project and solved it like this:

 Win32.DEV_BROADCAST_DEVICEINTERFACE deviceInterface;
 deviceInterface = (Win32.DEV_BROADCAST_DEVICEINTERFACE)
 string name = new string(deviceInterface.dbcc_name);
 Guid g = new Guid(deviceInterface.dbcc_classguid);
 if (g.ToString() == "a5dcbf10-6530-11d2-901f-00c04fb951ed")
 {*DO SOMETHING*}

I get the GUID and check if the devices GUID is the USB-GUID.