Rosbags containing no messages after reindexing

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Dear all,

I am working on a Ubuntu 20.04 system using ROS Noetic.

I was recording large-size rosbags via the rqt_bag GUI (recording images from multiple indsturial cameras). Each rosbag is between 500 MB and 14 GB. All the rosbags were recorded exactly in the same way using rqt_bag and always the exact same 8 topics were recorded.

Unfortunately, a lot of the recorded rosbags are damaged/broken.

When trying to play the rosbags I receive one of the following five feedbacks:

1. rosbag runs perfectly without any problem.
2. rosbag throws error: Unindexed bag. After calling "robag reindex" the bag was successfully migrated and the rosbag plays without problems.
3. rosbag throws error: Unindexed bag. After calling "robag reindex" the bag was successfully migrated, but when trying to play the migrated bag it plays the rosbag without error, but directly shows "No messages to play on specified topics. Exiting."
4. rosbag throws error: "Required 'op' field missing"
5. rosbag throws error: "Expected FILE_HEADER op not found"

I have no idea why there are five different results when recording the rosbag always in the same way.

During my search on the internet, I have once read that it could be based on the problem that the topics /rosout and /rosout_agg were not recorded. But e.g. in a rosbag where no errors appeared there were those two topics also not recorded.

I also already tried to repair the rosbag using the rosbag fix command, but no success, same problems.

Obviously, there is some problem with 'op' but I do not know and do not find what it is. Can we "open" the rosbag in any way to see what is written in the file? Because the file size is very big. There should be some data inside, but obviously they can't be read at the moment.

Any help to save the data would be really appreciated. Thank you very much.