• DriveInfo
  • Will not work on non-existent drives.
  • AvailableFreeSpace returns the freeBytesForUser.
  • TotalFreeSpace returns the freeBytes.

class Program
{
    static void Main(string[] args)
    {
        DriveInfo a = new DriveInfo("a");
        DriveInfo c = new DriveInfo("c");
 
        WriteDriveInfo(a);
        WriteDriveInfo(c);
    }
 
    private static void WriteDriveInfo(DriveInfo drive)
    {
        try
        {
            Console.WriteLine(string.Format("{0} ({1}): {2}", drive.DriveType, drive.DriveFormat, drive.VolumeLabel));
            Console.WriteLine(string.Format("{0} / {1}", drive.TotalFreeSpace.Humanize(), drive.TotalSize.Humanize()));
        }
        catch (DriveNotFoundException dex)
        {
            Console.WriteLine(dex.Message);
        }
    }
}
 
public static class SizeExtensions
{
    // Credits to http://blogs.interakting.co.uk/brad/archive/2008/01/24/C-Getting-a-user-friendly-file-size-as-a-string.aspx
    public static string Humanize(this long bytes)
    {
        double s = bytes;
        string[] format = new string[] { "{0} bytes", "{0} KB", "{0} MB", "{0} GB", "{0} TB", "{0} PB", "{0} EB", "{0} ZB", "{0} YB" };
        int i = 0;
        while (i < format.Length - 1 && s >= 1024)
        {
            s = (100 * s / 1024) / 100.0;
            i++;
 
        }
        return string.Format(format[i], s.ToString("###,###,##0.##"));   
    }
}

foreach (var drive in DriveInfo.GetDrives())
{
    WriteDriveInfo(drive);
}