CCTV surveillance is used in critical situations as proof, to find details, find specific people, vehicles, etc. Each CCTV records video, and through connecting interfaces, cables, or the Internet, they store it over a storage device. Some DVR cameras are also available with inbuilt memory in the form of SD cards.
So, it comes to Storage devices to retain this information. The period over which this data can be stored is prefixed and called a cycle. After the cycle completes, the oldest of the files start getting overwritten. As per the capacity of the hard drive, and the number of cameras, this cycle can be varied. So, if the cycle is for 15 Days for a 1TB hard drive, which is storing the recording of 10 DVRs, on day 16 files for 1st-day recording will be overwritten by 16th-day recording. Overwritten data is not recoverable. It is important to note that going throughout the cycle is not the only way to lose CCTV footage.