AL = Drive number (0=A, 1=B, etc)
        DS:BX = Disk Transfer Address (buffer)
        CX = Number of sectors to write
        DX = First relative sector to write
Return: CF = 1 if error
            AL = error code issued to INT 24h in low half of DI
            AH = same error codes as for INT 25h
Note: ORIGINAL FLAGS ON STACK!
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