obextreme

Hi all, i was wondering if anyone could help me with a problem i have with my CnM 264 DVR for some reason it wont allow me to back up any images i have. i have tried many usb hard drives as the instructions state but it keeps coming up with "no Usb Device" has anyone else had this problem and does anyone have any ideas on how to fix it ? your help would be greatly appreciated as my house was badly vandalised and the images are needed for the police thanks in anticipation