Time Machine couldn’t complete backup, untruely claims backup disk to be read-only

Nikolai Franke wrote:

Bizarrely, uninstalling Bitdefender did it—for now at least. No idea why, since I didn’t newly install it.

Not at all bizarre, based on the various add-on anti-malware and add-on security app experiences that get posted around here. The way these tools traditionally hook into macOS is often indistinguishable from how malware hooks in, too. That’s becoming more limited, though; that access is becoming more constrained.

Entertainingly, the built-in anti-malware has been blocking various attempts to corrupt macOS itself by add-on anti-malware. The built-in tools block the worst and most unfortunate of those efforts.

And FWIW, Avast surprised some folks a few years back. That happening with other add-on tools would not surprise.

Yes, seems the difference is elsewhere.

Are you using the same Partition for TM & saving files to outside of TM?

I can't remember where te change was, but Apple frowned inthat process eventually.

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

Use the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

Earlier versions of etrecheck...

https://macdownload.informer.com/etrecheck/versions/

You say you can copy files to that same Disk???

One note, the OS will try to mark failing drives as Read Only.

Try one of these two…

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx

Thanks to Jack-19…

It may be wise to run SMART Utility to check the drives' health.

Uninstall AntivirusforMac.app/Bitdefender, restart & run etrecheck again to see if there uninstaller left chaff hanging.

Did this problem only surface after 2024-01-25 installation of Duplicate Manager???

There are hunks of Avast, and at least one haxie, and some stuff has been crashing, too.