The best option is to format the DVR or NVR HDD. And, using Reolink cameras as an example, here's how to remove all video footage from a CCTV DVR/NVR:
Step 1:Connect your DVR/NVR to a monitor first.
Step 2: Go to the DVR/Device NVR's Settings after logging in.
Step 3: Locate the disc management option, and then format the hard disc drive to erase all of your camera's video and photo history.
Tips: After you format the HDD, your previous DVR/NVR setups, such as motion detection mode and email settings, will remain the same.
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