In Windows Mobile, external storage cards are usually represented as the '\SD Card' or '\Storage Card' folder, this varies from device to device. A simple way to enumerate the external storage cards is to check all directories under the root directory that have the temporary attribute flag set.

Here's a quick way to do so in .NETCF:

public static List<string> GetStorageCards()
{
    var list = new List<string>();
    var root = new DirectoryInfo("\\");
    foreach (DirectoryInfo directory in root.GetDirectories()) 
    {
        if (FileAttributes.Temporary == (directory.Attributes & FileAttributes.Temporary))
            list.Add(directory.Name);
    }
    return list;
}