What was in the September 24, 2020 update that killed my iMac?

I just did as well. (I didn't know that no one from Apple looks here so I ranted here...ooops!

👍

Whew!

Welcome to the club. The update took out my 2015 Macbook Pro. I hope yours is recoverable, mine isn't. All manor of troubleshooting, down to power manager reset, SSD format and clean install still leaves me with kernel task eating 600% and more, with the fans going nuts with both Mojave and Catalina.

I'm having the same problem. I allowed the update last night and today my system is crippled in so many ways. It's a miracle that I managed to get into this forum. Took a long time though. Some apps are running so slow as to be useless; some won't even open. Files I created yesterday and show on the Finder list (e.g., pdf), are not opening -- it says they don't exist. This is nuts. It's like the computer is drunk or on acid or something. I ran a diagnostic and it just ended with a green checkmark with no details.

Coincidentally I updated my IOS to 14.0.1 yesterday, it wouldn't sync my I phone so I was guided to download some software, the iPhone synced perfectly to my ITunes and now my Garage Band has been killed. It won't allow me to install Garage Band so I am stuck possibly with the only option of upgrading to Mojave 10.14 and get a more up to date version of Gargage Band. Why have Apple products gone so terribly wrong?

Thank you Starplot and also to the many here reading and helping. There is something definitely fishy and odd about this, your experience is pointing in the same direction more or less as my experience. I too am using 6.0.5 Garage Band, which has been very reliable and stable. The prospect of losing my own created sounds isn't one I wish to contemplate so yes Apple, bloody well get on and fix it!

There must be something screwy in this last update... I had a similar experience.

I installed the update and immediately my 21.5" (late 2013) iMac became unusable (beach balling on every mouse click; my Time Machine backup disk showed NO backups had been made; EtreCheck showed MAJOR issues; etc., etc.)

I spent several hours running Apple H/W diagnostics, booted into Recovery Mode and ran Disk Utility on both my Macintosh HD and Time Machine backup disks;. No H/W errors or disk errors were reported.

I then used Recovery Mode to reinstall my last Time Machine backup image just before I installed the last Security Update, and now everything is operating normally again.

I have NO third party S/W installed on my iMac -- I have no idea why this particular update "bricked" my system.

My Late 2013 21.5inch Mac had been running slow over the last 6 months (frequent beach ball). I don't have automatic updates switched on, but overnight on Sep 25th, my Mac updated to Safari 14 and then everything went super slow and got even slower with Apps not loading and system monitor showing incredibly heavy disk usage.

Rebooted several times, but got even slower and finally (after 5 hours of trying different solutions) my Mac completely bricked and wouldn't get past the apple logo.

So, did a full recovery from my time Machine (shift R during boot and selected a backup prior to the update), with fingers crossed as I hadn't done this before.

Thirteen hours later ...... and my Mac is super fast and fully functioning.

First thing I did, was turned off automatic updates, as I don't want to be stressed out again.

Some oddities during the recovery ...

• my Photos library had to be repaired, which was done automatically
• my emails had to be converted to the 'New' Mail software (according to Apple) !! - they are all there.
• some old recurring events ended up in my Calendar, but these have since removed themselves automatically !!

Conclusion - Apple don't test their MacOS updates on old machines.

I had the same issue, my iMac Mini was not working anymore (Late 2012 edition). I will not upgrade to Catalina, because then some of the apps I need won't work anymore because no updates are provided in the latest Mac OS.

Fortunately I could recover everything via the recovery mode without deleting my personal data. But now I have a build from September 2019 - 2019-05 (18G103).

How can I upgrade again to Mojave build 2020-04 (18G6020)?

Same here, as reported in a similar thread. But by some miracle, I managed to reinstall the Mojave OS from a Time Machine backup, and now more or less everything seems like normal. However, I flatly refused to "obey" when the most recent request to install this Security Update appeared in an information box, and I won't install it before Apple has confirmed that the update is now bug-free.

I have never before had such a horrifying experience with Apple (been a user since 1988). Please Apple, stop inventing hundreds of new gimmicks all the time and concentrate on the important things. Most people out here NEED their computers permanently!