batch mit diskpart schlägt manchmal fehl warum? und wie abfangen?

@echo off
setlocal ENABLEDELAYEDEXPANSION
:: Full path to diskpart.exe. Defaults are:
:: Windows 2000: "C:\Program Files\Resource Kit\diskpart.exe" 
:: 2003/XP:      "C:\windows\system32\diskpart.exe" 
set dp=c:\windows\system32\diskpart.exe

:: Volume label
set label=backup

:: Temporary command file for diskpart.exe
set dps="%TEMP%\dp.txt"  

echo list volume>%dps%
echo exit>>%dps%
set label_short=%LABEL:~0,11%
if exist %dp% (
    for /f "delims=" %%i in ('%dp% /s %dps%') do (  
        set string=%%i
        if not "!string:%label_short%=!"=="!string!" (  
            set volnum=!string:~9,3!
            set volnum=!volnum: =!
        )
    )
    if not "!volnum!"=="" (  
        echo Volume Label:  %label%
        echo Volume Number: !volnum!
    ) else (
        echo Cannot find volume with label %label%
    )
) else (
    echo Cannot find %dp%&goto :EOF
)