Little more info required befor some one could help you,
What Dvr do you have? What size of USB are you using and what format is it?
Reason I ask that is depending on the age of the Dvr it might not see a USB drive over 2GB, or if your USB is over 32GB it will be formatted to ntfs, again a lot of dvrs don't like nyfs formatted drives so I always make sure any external USB drive I use for cctv recovery is formatted to FAT32.
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