How is a flash drive (usb thumb drive) formatted? I have a dvr as part of a security system. It has a usb port to download video to a thumb drive. I've been trying to use a usb portable hard drive instead so I can download days of video vs hours of video. A 4gig thumb drive works fine, but is limited in time. The system doesn't recognize the hard drive. I've tried formatting the hard drive with and without a drive letter. Any suggestions from the experts?

It might only recognize drive formatted in FAT or FAT32 with a max of 4 Gb. Try formatting the drive in FAT32. That will mean multiple 4 GB or less partitions on the drive, and the DVR might only see the first partition.