The best way to do this is to setup a batchfile called erase.bat that looks similar to this.
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@echo off
REM Change to the directory in which ARCserve is installed.
REM The "" marks are necessary if it is installed in a directory with 2 words.
cd "C:\Program Files\ComputerAssociates\ARCserve"
REM run the command to erase the tape
REM -z tells it to erase, -m1 tells it to only erase media label. There are two other options -m2 and -m3. They are the same as the erase options at the gui level.
tapetest -z -m1
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If you add this batchfile to the pre/post section, it will erase the media label before the backup starts and you will always have the correct date for every backup.
I hope this helps,
Eric