Most DVRs use proprietary interfaces and filesystems which makes their recovery more complex.  Even in minor cases where cloning a drive is possible, it will do you no good unless you have an identical DVR to plug it in on to read and view the data. Therefore in most cases, it is a two pronged problem since you not only have to overcome the failure in of itself but also convert the data into a usable format for you the client to read and access.