Resolved Data Recovery Question

[Rhobes]

Hi All-

I trashed (emptied trash) some MP4 video files and recovered them using Stellar Phoenix software. All but a couple short videos are corrupted and stop playing after a few seconds. Question I have now is: Does the process of emptying the trash on an iMac cause the corruption to the files (all previously fine)?

I'm assuming the recovery software I just bought is not corrupting the files on recovery, although, funny they sell a video file repair software separately. All the .mp4 files seem to stop on a frame about 2-4 seconds into the videos yet the progress bar keeps moving to the end of video. Another funny thing, the preview feature of repaired MP4's using the trial (free) version of the Stellar video repair software only shows the 1st 2 seconds of the "repaired" file. Of course, the 1st 2 seconds play cause all the files play for at least 2 seconds before they stop. Makes me question it all.....

Any comments welcome-

 

[grahamperrin]

Without getting too technical: after the spaces occupied by a file are marked as free, then you have no control over what the computer will do with those spaces.

At least with rotational media (hard disk drives): if the spaces previously occupied by a file are towards the 'fast' area of the disk, then it's likely that OS X will soon overwrite a part of one of those spaces. "… Stop using the disk … as soon as possible.…" and so on; that's not an endorsement of Mac Data Recovery Guru, it's just sound advice.

In simple terms: you might not feel lucky, but you are lucky to have recovered anything.

 

[Rhobes]

Without getting too technical: after the spaces occupied by a file are marked as free, then you have no control over what the computer will do with those spaces.

At least with rotational media (hard disk drives): if the spaces previously occupied by a file are towards the 'fast' area of the disk, then it's likely that OS X will soon overwrite a part of one of those spaces. "… Stop using the disk … as soon as possible.…" and so on; that's not an endorsement of Mac Data Recovery Guru, it's just sound advice.

In simple terms: you might not feel lucky, but you are lucky to have recovered anything.

So, I read somewhere that the oldest trashed items would be overwritten 1st, meaning that those items I just trashed would be safe for a long time on my 2TB HD, but, from your description that most likely is not the case. I also read that not using the computer period would be a safe thing to do, but I found that out a few hours after the fact.

Since I had been using the computer to research what to do I logically thought probably O.K. as long as I didn't trash anything- for what that thought was worth. Never-the-less, I'm inclined to think my little usage was enough to corrupt those MP4's (110 of them).

I have saved what I got to an external drive now. I then went back to the iMac and opened FCPX and imported a couple of those corrupted videos. Right where the video stops playing and ends stuck on a frame there is absolutely nothing beyond that frame, just black space, no frames, no audio there. I did find the large majority of the mp4s (96 of them) in a cluster together, here and there a different video from who knows where, but mostly grouped together. So, for the most part all the files had several seconds of frames with audio at the beginning then all the remaining empty space.

 

[grahamperrin]

… I read somewhere that the oldest trashed items would be overwritten 1st …

It's not orderly in that way, but time is a factor.

After you empty the Trash: the longer that you allow the operating system to use the file system, the greater the likelihood that a write of a file will occupy a space that is marked as free.

Binary: a space is either used, or free. There's no in-between state (the systems are not designed to tell whether a space was recently used).

… I'm inclined to think my little usage was enough to corrupt those MP4's …

Think of each recovered file as incomplete.

… the files had several seconds of frames with audio at the beginning then all the remaining empty space …

Visualise a single, complete MP4 occupying multiple spaces; all of one space in one area of a disk plus part of another space elsewhere. (In different areas, because it was quicker for the file system – HFS Plus – to write to those two spaces, than to spend time seeking/reaching an area where a single space was large enough.) Technically (in HFS Plus terminology), two extents to that one file.

After the trashing of that MP4: a write of a single file, by the system, might overwrite all or part of either extent. Might overwrite both extents.

I don't know the internals of MP4 … but I guess that if it's in two extents, and if two 'ends' (for want of a better word) of an MP4 are examined in isolation, only one end – maybe the start – will include what's necessary for the extent to be recognisably from an MP4.

 

[Rhobes]

It's not orderly in that way, but time is a factor.

After you empty the Trash: the longer that you allow the operating system to use the file system, the greater the likelihood that a write of a file will occupy a space that is marked as free.

Binary: a space is either used, or free. There's no in-between state (the systems are not designed to tell whether a space was recently used).



Think of each recovered file as incomplete.



Visualise a single, complete MP4 occupying multiple spaces; all of one space in one area of a disk plus part of another space elsewhere. (In different areas, because it was quicker for the file system – HFS Plus – to write to those two spaces, than to spend time seeking/reaching an area where a single space was large enough.) Technically (in HFS Plus terminology), two extents to that one file.

After the trashing of that MP4: a write of a single file, by the system, might overwrite all or part of either extent. Might overwrite both extents.

I don't know the internals of MP4 … but I guess that if it's in two extents, and if two 'ends' (for want of a better word) of an MP4 are examined in isolation, only one end – maybe the start – will include what's necessary for the extent to be recognisably from an MP4.

Well, it is interesting. May those incomplete files rest in peace....
Thanks for your comments Grahamperrin-