I believe this DVR would be my best recommendation as it can be used with your current cameras. Would allow you to upgrade to HD cameras and would allow you to schedule backups to the second drive.
Unfortunately USB just isnt fast enough to keep up with regular recording but it could be backed up to it. Additionally you could configure an FTP backup but our tech support does not support that feature.
http://www.securitycameraking......-prd1.html
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