I contacted $vendor about the drives from the DVRs - They remoted into a system we'd set up with the software from the OEM - was able to see the drives, and the files, but playback failed.They used the most recent version of the software and it was indicated by them that the video files were corrupted.

They also suggested two older versions that didn't see the files at all, but detected the drives. And that these may be AVI files

Vendor suggested searching online for recovery for $brand DVRs - which indicated they may be "DV4" files.

The drive themselves seem accessible (indicating there's no file system corruption) - they're a linux/ext2 format and they were visible from a third party, read only file system loader from windows. I used the read only software to copy a subset of the files for analysis.

There are a set of files with similar names - one file with a I extension, which appears to be a password protected SQL file and a set of files with the extension .stream.

I attempted a preliminary analysis of a set of .stream files I'd copied out to my laptop with "trid" - a forensics/analysis software used to identify file types. It identified the video files as mp3s (which is an audio, not a video format). I also attempted to play the individual copied out files with VLC - a common video player that supports most common video formats. I did so both with the .stream extension and with a renamed extension.

Most file recovery services tend to focus on hardware or filesystem failure - I was unable to find any cases/literature of forensic recovery from these machines - so I'm unable to recommend a method or possible vendor for recovery of these. .