Automatic Catalina upgrade fails to complete its task, over and over

When you run First Aid click "Show Details" and manually scroll back through the report to see if there are any unfixed errors are listed. Unfortunately there can be catastrophic file system issues even if First Aid reports everything as "Ok".

Try installing the updates while booted into Safe Mode. If this works, then you have some third party software installed which is interfering with the normal operation of macOS. Typically this will be caused by anti-virus apps, cleaning apps, and third party apps. You should uninstall these types of apps by following the developer's instructions since these types are not needed on a Mac and usually causes more problems than they solve plus they impact system performance.

Lucar21 wrote:

1. I uninstalled ClenMyMac from within the app as you kindly suggested and check for third-party apps. I have a few ones like, adobe, Skype, etc., deemed as Identified Developer, which I assume are allowed by Apple. I did not find any Unidentified Developer in System Information.

Adobe products are fine (except for Flash). However, just because an app is from an "Identified Developer" does not mean the app is without risks or problems. There are a lot of apps that Apple allows to be installed from third party developers, but you must still do your own research to ensure those apps are safe to use. CMM is considered by most contributors here to almost be a form of malware. I'm not a macOS software expert, but if you run EtreCheck and post the report here using the "Additonal Text" icon so the other more knowledgeable contributors can inspect the report to see if you have any apps installed that may interfere with the normal operation of macOS.

2. I started on safe mode and allow automatic installation of the upgrade which again failed to complete.

3. Still within Safe Mode I run Disk Utility on Macintosh HD (disk1s5) and Macintosh and this is a copy of the Show Details report. It appears that something is happening with the HD but I cannot understand the problem. Please help.

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Running First Aid on “Macintosh HD” (disk1s5)+

Checking snapshot 1 of 1 (com.apple.TimeMachine.2021-09-18-041203.local)

error: (oid 0xaf7d9) apfs_extentref: btn: dev_read_finish(718809, 1): Input/output error

Snapshot is invalid.

The volume /dev/rdisk1s5 could not be verified completely.

Try booting into Internet Recovery Mode using Command + Option + R to run First Aid again to see if the file system can be repaired. If it is repaired, then try running First Aid again while booted normally into macOS to verify that the error has indeed been fixed by manually checking the report again both times.

Since the issue is with one of the APFS snapshots you can try to delete that snapshot to see if it resolves your problems. Here is an article with instructions:

https://derflounder.wordpress.com/2018/04/07/reclaiming-drive-space-by-thinning-apple-file-system-snapshot-backups/

Otherwise you will need to erase the whole physical drive and restore from a backup or clone. To erase the whole physical drive will destroy all data on the drive that is not backed up. To be able to see the physical drive you must click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. The internal physical drive should be the top most item listed under "Internal" and is usually identified by the make & model of the drive such as something like "Apple SSD....".