you can use "/" on the ODS task from ePO to scan all drives.
1) is there a log on the Mac of the On demand scan?
2) is there any way on the epo server to see how many files etc were scanned?
Locally you can look at the Dashboard of McAfee Security for Mac to see the ODS details.
Or you can look at /var/log/McAfeeSecurity.log for ODS details.