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Formerly reliable TimeMachine failing under Big Sur


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TimeMachine has been failing backing up to a hard drive that worked flawlessly since High Sierra on an Intel machine. I didn't "introduce" that backup to my new M1 Macmini until it was a couple versions into Big Sur, maybe 11.2. Tried to backup many times, and miraculously with the 11.4 upgrade, it started working again. After several days of successful backups, it returned to the generic "TimeMachine couldn't backup to 'External'. An error occurred while preparing the backup" and continues through 11.6.2
I've run TimeMachine Mechanic, but it doesn't show me anything specific (I'm not clear how to obtain anything useful with this tool). When I run Disk Repair on it, it takes a half hour to process, and then reports everything is okay.
Suspecting it might corrupt original data, I ran Disk Repair on my internal HD (all is good), and even dug up another external hard drive to become my TimeMachine backup destination, and that backed worked fine -- so I have to conclude that the original data is ok.
It's likely that I could reformat this drive and it would probably work....HOWEVER.....while the situation is fresh, I'd like to understand the failure better, and see if there's a fix that doesn't require reforrmatting as Apple's article concludes < >
Of course my backup drive is formatted as HFS+ since it started life before APFS was required or advisable. Several questions.
1) I know in the Big Sur betas in Disk Utility, there was a choice to "convert to AFPS", which I'm guessing was a non-destructive way to convert a hard drive. That command is no lpnger available in the current Disk Utility. Does that function still exist somewhere else? Is there a command line way to invoke it?
2) In the case of a hard drive that has been employed for TimeM backup for awhile, but still has plenty room, is this just expected behavior? In other words, once you've upgrade to Big Sur (or Monterey), is it just a good idea to reformat your TimeMachine hard drive to APFS?

3) Could it be something in moving my backup from an Intel machine to Apple Silicon?
This topic was modified 1 month ago by appleseed
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According to Howard Oakley’s blog at timemachine will work better when the destination is APFS. This means you first have to reformat your destination disk to APFS. 

@cpragman Yes, I understand that's APFS is better. The questions, what's the cause of my usually reliable system's failure, and is there a way to get from HFS to APFS without reformatting?