munenomura
I take it you have created your bks file, where you have selected what to backup.
I have found that to get a reliable backup I have to use few more switches. Here is an example of my backup batch file that I run every day. It should work fine for you aswell with a few changes.
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REM this to prevet problems with the media datapool you REM need to copy sleep.exe to \winnt\system32
net stop ntmssvc
sleep 60
net start ntmssvc
sleep 60
REM you need to change this to your backup device
rsm.exe refresh /lf"HP C5683A SCSI Sequential Device"
sleep 60
for /f "Tokens=1-4 Delims=/ " %%i in ('date /t') do set dt=%%i-%%j-%%k-%%l
for /f "Tokens=1" %%i in ('time /t') do set tm=-%%i
set tm=%tm::=-%
set dtt=%dt%%tm%
C:\WINNT\system32\ntbackup.exe backup systemstate "@C:\backup\BACKUP.bks" /n "Nameyourbackuphere %dtt%" /d "Nameyourbackup here %dtt%" /v:no /r:no /rs:no /hc

n /m normal /j "%dtt%" /l:s /p "4mm DDS" /UM
REM you have to change /p to your backup device name
rsm.exe eject /PF"Nameyourbackuphere %dtt% - 1" /astart
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That set of switches works with me and is very reliable.
this is what the switches do for you
Systemstate "Specifies that you want to backup system state data registry etc."
bks filename "specifies the name of your backup file remember to change to your path"
/j {"job name"} "name your job in the backup log"
/p {"pool name"} "specifies your media pool where you want to use media"
/v:{yes|no} "verify your data after backup"
/r:{yes|no} "restricts access to this tape for owner or members of the Administrator group"
/l:{f|s|n} "specifies the log file f=full, s=summary, n=none"
/m {backup type} "specifies backup type has to be either: normal, copy, differential, incremential or daily"
/rs:{yes|no} "backup removable storage database"
/hc:{on|off} "use hardware compression if available"
/um "this is the important switch!. Find the first available media, formats it and uses it for the current backup operation. It has to be at the end of the line and should not be enclosed in quotes.
If you have an Exchange server you need to add two more switches.
/ds {"Server name"}
/is {"Server name"}
I hope that explained it...maybe I should write a proper faq about this but someone probably already have.