Errors using pre/post commands

Hi at all,
i've a problem with a backup that must use network drives as input and as output. if i connect drives manually every works fine, but if i would to make the backup using scheduled job i've problems.
The first is that network drives could be disconnected (i don't know why but it is). Then i say: ok, i an use pre/post commands and a net use..

i look at http://kb.acronis.com/content/1742 and i set a pre command using a batch like
net use z: \\source_server\c$ /user:username pwd

and a post like
net use z: /delete /y

commands test are positive, but when i start the job (manually or scheduling) i receive error messages:
1 Informazioni 23/06/2011 11.26.02 Operazione "Ofiuco DB incr" avviata.
2 Errore 23/06/2011 11.26.02 Impossibile trovare il file o la cartella 'Z:\Programmi\Microsoft SQL Server\MSSQL.1\MSSQL\Backup\Autenticazioni\'.
3 Errore 23/06/2011 11.26.02 Impossibile trovare il file o la cartella 'Z:\Programmi\Microsoft SQL Server\MSSQL.1\MSSQL\Backup\AutenticazioniTest\'.
[..]
19 Attenzione! 23/06/2011 11.26.03 Priorità modificata in Bassa
20 Informazioni 23/06/2011 11.26.03 Processo di distribuzione terminato con codice 0
21 Informazioni 23/06/2011 11.26.03 Esecuzione del comando utente completata: C:\tmp\ofiuco_on.bat
22 Errore 23/06/2011 11.26.06 Nessun elemento da archiviare. (0x40011)
Tag = 0xBA19E88DC5FE9FFA
23 Errore 23/06/2011 11.26.06 Operazione completata con errori.

it seems that the process looking for the archives to backup does not wait for the end of batch command execution. why? any suggestion? thanks