I wrote a small test for the event aggregator and I believe the problem is in this line:
DriveInfoManager driveInfoManager = new DriveInfoManager(mockContainer as IUnityContainer, mockEventAggregator as IEventAggregator);
I tried this and was able to get it to work:
DriveInfoManager driveInfoManager = new DriveInfoManager(mockContainer.Object, mockEventAggregator.Object);
You will still need to execute this lines:
var mockEvent = new Mock<MonitorDrivesEvent>();
mockEventAggregator.Setup(e => e.GetEvent<MonitorDrivesEvent>()).Returns(mockEvent.Object);
This will only work if
DriveInfoManager only needs MonitorDrivesEvent event. If it requires anything else, you will need to mock it also.
Hope it helps!