Hi Giovanni,
The alternative to scripted NTbackup is to use BackupAssist.
Much simpler because all you do is in the backup destination section, you can add a %Medialabel% variable in your destination path to your External HDD.
Medialabel variable gets replaced by the "Day of the week".
This means that you get:
Monday-Backup in extern HDD in E:\Backup\Monday\xxxxx
Tuesday-Backup in Extern HDD in E:\Backup\Tuesday\xxxxx
Hope that helps.
S.C.