If the box can record by itself then you should find a hard drive in their... They do vary in size though...
like the one I got before this one was only 80 GBs... But the one I get from this DVR is a decent 320 GBs maybe not the best size on the market but what can I argue..? It was completely Free! And never better models of DVRs have even larger hard drives in them just waiting to be plundered!!
Time to get some practical use out of some otherwise old Trash, so grab your Specialized Star Bits and lets get cracking!!
Also if plugging the Hard Drive into your computer doesn't cause it to instantly work, please stick around as in the second half of the video I explain how to fix that... (In Windows)