huge discrepancy between size of time machine backup and content of fusion drive

I have used three different ways to calculate the amount of storage that should be backed up by Time Machine.

First, there is the backup of the contents of my MacBook Pro (from which I migrated to an iMac two weeks ago).

Second, I checked the "Estimated size of full backup" in the Options panel of Time Machine's Preferences.

Third, I did a "Get Info" on a large number of folders that are high up in the hierarchy of the folder structure on my Fusion Drive.

I did these calculations last week and again three or four days ago, with the same, already known, result: almost 100GB is missing on the Time Machine backup disk. However, yesterday I had some free time and tried to acquaint myself with the new "Podcasts" app and how all the hundreds of podcasts that I had collected and saved on my hard disk (using iTunes) during the last 14 years or so, were migrated to this new environment. I found out that all podcasts had been moved from their previous (and still existing but now empty) folder structure under the iTunes Media folder, to a new folder:

~/Library/Group Containers/243LU875E5.groups.com.apple.podcasts

When I checked out this new folder, I noticed that all the podcasts had been gathered (without any organised folder structure) into a sub-folder called "Cache"...

Knowing how Time Machine ignores "caches", I immediately checked if this folder and all the podcasts it contains, existed on my Time Machine backup drive. To my surprise this folder, including its subfolders, was indeed backed up but without any of its podcasts!

In other words: the folder and subfolders of ~/Library/Group Containers/243LU875E5.groups.com.apple.podcasts are backed up by Time Machine, but the contents of the "Cache" folder inside this structure, meaning the podcast files themselves, are not backed up! In my case this amounts to over 80GB and does explain part of the huge discrepancy I talked about in my original post.

I find this very problematic, because if ever the 80GB of podcasts that I collected over all these years, would get lost or deleted by accident, there is absolutely no guarantee that I can download any of these podcasts again. And I'm not even talking about the time it would take to download all of them once again. Where I live the download speeds are ridiculously low! Podcasts saved locally should be backed up by Time Machine like any other file within the Users folders and subfolders!

Mr leroydouglas,

I presume you haven't yet understood how serious this "issue" is. It's a huge problem considering the fact that Time Machine is no longer reliably backing up everything it needs to back up and tells the user it is going to back up (the "Estimated size of full backup"). Perhaps losing 100GB of content of *your* internal drive is not an issue for *you*, but considering there is almost 15 years of work on *my* fusion drive and Time Machine is no longer backing it all up, I see this as a serious bug inside an OS that millions of us are using and have to rely upon.

That sufficiently answers your first question. Your second question puzzles me even more. Of course, Time Machine "missing" 100GB out of 300GB to be backed up, has a serious impact on the initial backup because that one and all consecutive, incremental backups, are simply incomplete and can thus not be relied upon. In other words, Time Machine has just become totally useless.

I must admit that your comment that "it is best to let Time Machine do its thing", made me laugh out loud. I used to work as a support engineer for one of the biggest software companies out there, and I can assure you that letting software programs ("apps" as they are now so cutely called) do their own thing without checking that they are doing that "thing" consistently and reliably, was the mother of all computer disasters and often also of the loss of years of hard work.

I didn't try and "second guess" the "magic behind the curtain". I followed a series of facts and used logic to come to the conclusion that Time Machine is not backing up the whole drive of my iMac, missing one third (!) of the "Estimated size of full backup". There is no such thing as magic in software, but I will admit there are nowadays many curtains (way too many) to hide serious mistakes from being taken care of.

Again, you are mistaken. I do not "think" that Time Machine is missing 100GB, I have actual and factual proof, looking at the total size of all files on my Fusion Drive and the total size on Time Machine's backup drive - telling me there is 100GB less on the backup drive. And no, this is not due to cache files or such, I have checked everything out long before I posted about this huge problem. And actually, yes, I did start with a newly formatted backup drive. And no, I'm not going to start anything anew because I have work to do and debugging software made by Apple is not part of that, I'm afraid.

Your links have absolutely nothing to do with the problem I posted about. And thank you for giving me advice not to rely on Time Machine as the only backup solution. You will understand that I'm not a beginner in computing and that I do make a bootable copy of the internal drive of my computers at regular intervals - however not every hour, nor every day.

In short: I need Time Machine to backup everything it is supposed to back up, and the huge discrepancy of 100 (one hundred!) GB needs a far better explanation than "it's Time Machine doing its magic behind the curtain". I don't need a magician, I need an operating system that works flawlessly, especially when it comes to creating and managing backups.

FYI: Erasing and reformatting the external Time Machine backup drive (for the second time) did not solve the problem. I am still missing around 100 GB that is present on the internal Fusion Drive but is not being backed up. The "Options" panel in the Time Machine Preferences calculates the size of the full backup correctly but the initial Time Machine backup "misses" 100 GB and subsequent incremental backups do not include those 100 GB either.