Make sure to disable the Hard Disk Sleep feature of the Energy Saver software by setting Hard Disk Sleep to "never" in the Energy Saver control panel before testing. Otherwise, the drive may go to sleep in the middle of testing, which will cause the freezing or test failures you are experiencing.
Make sure that the drive has the latest version of the Apple hard disk driver, and any firmware updates that may be required for the drive, as these may also cause these types of symptoms.