Contact Backblaze support and see what they suggest. They may be able to point you to log files that offer more information on the reason for the safety freeze.
I did follow up, but haven't gotten a response yet. Will update as I get it.
Edit: New update:
> I apologize, but this would in fact be the only sure fire way if you are concerned about any deleted files. There wouldn't be a way to compare hashes the way you describe in this case as that mechanism is simply not implemented into the Backblaze software. The Backblaze software is intended to prevent data loss. It would not be intended to be able to automatically cross reference local and server data against each other to display what is and is not backed up on our servers.
Would you mind posting the exact information you gave them, and their response - redacting sensitive information of course.
This was my message to them:
> Hi, I was safety frozen. How can I find out the cause? I haven't made any significant changes to my system lately. In the support document it says that I should check if any data is missing - I have a lot of data backed up in backblaze, how can I know for sure nothing is missing or corrupted? How should I proceed? I've checked with CrystalDiskInfo and it seems that all SMART data is fine. Computer behaves normally. I've attached bzlogs and bzreports folders, just in case they're important.
And this was their response:
> Unfortunately there is simply no way to confirm what caused a safety freeze from our end. We can only provide the most common causes in this case, however we wouldn't be able to pinpoint the exact cause. The only way to verify any missing/deleted data would be to access your View/Restore Files page and cross reference what is found on our servers and what is found locally on your system. There wouldn't be any direct or automatic method of checking what data is missing, if any.
Edit, New Update:
> I apologize, but this would in fact be the only sure fire way if you are concerned about any deleted files. There wouldn't be a way to compare hashes the way you describe in this case as that mechanism is simply not implemented into the Backblaze software. The Backblaze software is intended to prevent data loss. It would not be intended to be able to automatically cross reference local and server data against each other to display what is and is not backed up on our servers.
He asked me what would happen if he put millions of very small files in a directory, I explained about filesystems and that each file would take up a 4k sector and space in the file, and that the behavior of the shell is undefined if there are more than 10,000 files in a directory as defined by POSIX...
I'm pretty sure he tried it anyways :-)
I had a Mac which crashed and that had a whole bunch of files go missing. Backblaze didn't want to do backups with a huge subset of my disk files gone without having gotten notification messages from the file changes API that this stuff should be missing/was deleted. So they safety froze my machine.
It's cool that Backblaze notices this. But yes, the question of what to do next isn't clear. Does one just start a new backup? Does one scour the backup looking for anything that may be missing on your machine, now? Etc.