The most common issue that causes the DVR+(CM-7500GB16) not to record, have choppy video and behave strangely is usually a defective or unsupported hard drive.
January 2017, we only recommend 1 External Hard Drive. Toshiba ProductID: HDTB310XK3AA
Many of the shelf hard drives will work fine with the DVR+(CM-7500GB16) but it's not practical for Channel Master to test every single drive on the market for Ideal performance so we strongly suggest the hard drives that we recommend. You may try any other drive at your own risk, but the downside is, if it does not work, you may need to try another one.