@Paxboy and Holdatcharlie:
Whether data is recoverable from the digital CVRs depends on a number of factors. If these were normal magnetic media drives, then it would be likely that anything that hadnt been overwritten would be retrievable but also its likely it would need a lot of work to actually get anything constructive out of the data.
However, there are issues if the CVR drives were solid state drives (SSDs). Anyone with any clue of computer forensics should know you cant just plug them in and image as you would normal magnetic media drives. SSDs have a garbage collection function that can run independently of any software, all it needs is to be powered on so your drive can be deleting data in the background faster than your imaging software can collect it. Not good for a computer forensics examiner having to explain why you havent got any data at all.
First question is if SSDs are used in CVRs does the CVR programming have the garbage collection process running in the background while its writing data to the disk to free up disk space? If so, then all you are likely to retrieve is the current length of the recording. If not, then you may be lucky to get partial bits of the recording being overwritten although all that may get you is silence in this case.
Also, one would like to think those designing systems using SSDs are aware of the reliability issues that are being raised about them in certain situations (e.g. power failure). If power fails it can make the drives inoperable and even those supposedly guaranteed against power failures do not seem to live up to their stated reliability looking at some of the research going on.
I dont know much about the CVRs but do they have alternative power sources or backups in case of drive failure during the flight?
Some articles for the techies on the issues with SSDs and data retrieval:
Solid State Drives and Forensic Troubles - WP's Police Tech
The mysteriously disappearing drive: Are power outages killing your SSDs? | ExtremeTech