int i;
            byte[] buffer= new byte[512];
            int offset=int.Parse(textBox1.Text);


            try
            {
                FileSourse = new FileStream(label4.Text, FileMode.Open);
                FileDest = new FileStream("\\\\.\\A:", FileMode.Open);       !!!!!!
                textBox1.Text = FileSourse.Length.ToString();
                
                // Читаем в буффер и записываем на дискету
                for (i = 0; i < int.Parse(label6.Text); ++i)
                {
                    FileSourse.Read(buffer, 0, 512);
                    FileDest.Write(buffer, 0, 512);
                    FileSourse.Seek(512, SeekOrigin.Current);
                    FileDest.Seek(512, SeekOrigin.Current);
                }

            }
            catch (IOException exc)
            {
                MessageBox.Show(exc.ToString());
                return;
            }

V>Хотя может я и не прав

int WriteSector(char sector, char cylinder, const char *p_buffer)
{
    char headnum=0;
    char drivenum=0;  // always 0 (A:)
    
    asm{
        mov ah, 03h             // write disk sectors from memory
        mov al, 01h              //  number of sectors to read/write (must be nonzero)
        mov ch, cylinder        // cylinder number (0..79).
        mov cl, sector            //  sector number (1..18).
        mov dh, headnum            // head number (0..1).
        mov dl, drivenum        // drive number (0..3 , for the emulator it depends on quantity of FLOPPY_ files).
        mov bx, p_buffer        // ES:BX points to data buffer.
        int 13h
    }            
        
    Test();
    return 0;
}

на чистом C#?

The .NET Framework does not support direct access to physical disks through paths that are device names, such as "\\.\PHYSICALDRIVE0 ".

int Write2Sector(const char *FileName, int SectorTo, char *buffer)
{
    char buffer[512];
    int rd;

    DWORD le;
    HANDLE hFile = CreateFile (_T("\\\\.\\A:"), GENERIC_WRITE, FILE_SHARE_WRITE, NULL, OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL, NULL);

    if (hFile == INVALID_HANDLE_VALUE) {
         le = GetLastError ();
    }
    BOOL res = SetFilePointer (hFile, 0, NULL, FILE_BEGIN);

    DWORD written;
    res = WriteFile (hFile, buffer, 512, &written, NULL);
    
    CloseHandle (hFile); 


    return 1;
}

DNN>int Write2Sector(const char *FileName, int SectorTo, char *buffer)
DNN>{
DNN>    char buffer[512];
DNN> //<skip>
DNN>}
DNN>

using System;
using System.Runtime.InteropServices;

class Program
{
    [DllImport("fddio.dll")]
    static public extern int Write2Sector(string FileName, int sectorTo, byte[] buffer);
    
    static void Main()
    {
        byte[] buf = new byte[512];
        for(int i=0; i < buf.Length; i++)
            buf[i] = (byte)i;
        
        Write2Sector(@"\\.\A:", 0, buf);
    }
}