The Drive Space Monitor uses RPC or SNMP communication to test the amount of available drive space on a specified drive, share or mount. The Drive Space Monitor Wizard allows you to configure Drive Space Monitors quickly and easily. However, if you prefer greater control over the process, you can Clone an existing Drive Space Monitor and make any required configuration changes manually.
Do you know which version of ipMonitor is running on the 'old instance?' How are you initiating the scans in each instance?
If this information isn't helpful and you still can't get it figured out, I would recommend opening a support ticket; they should definitely be able to help you get it configured.
HTH,
Craig