Is it that you cannot read the movie files or you cannot read the flash drive?
The movie file formats are pretty universal nowadays, most will be either avi, mpeg or quicktime and you can get viewers for all of those on any system.
Now, the file system on the drive may be specific to one OS or another, so that is a bigger problem. There seem to be tools out there to read files from linux partitions in Windows, so I would look into those:
https://www.pendrivelinux.com/accessing-a-linux-ext2-or-ext3-partiton-from-windows/
The test flash drive we used was reformatted by the dvr before copying. After copying our Windows machines didn’t even know a flash drive had been plugged in. They didn’t even see it.