Recovering from a bad update

My Mac hasn’t been the same since I installed Mojave Security Update 2020-005. Its fans run up shortly after starting up, it seems to be forever short of memory, and much of the time it’s slow, too. What’s wrong and how can I fix it?

That particular Security Update had these effects on many of those who installed it. Sometimes simple solutions can resolve update issues quickly, such as resetting the SMC and the NVRAM. Starting up in Safe mode, with the Shift key held down, leaving your Mac a minute or two so that it clears its caches, then restarting back into normal mode, can also sometimes work.

In this particular case, for those Macs most severely affected the only solution was to revert to Mojave as it was before that update. If you have a Time Machine backup running and can do this within 24 hours of the update, you may be able to roll back to the last snapshot made beforehand. Restart in Recovery mode, select Restore from Time Machine Backup then choose your startup disk, and macOS will inform you if there’s a snapshot available. Third–party backup apps which make snapshots, such as Carbon Copy Cloner, are an excellent alternative here.

If you still can’t fix a bad update like this, the only solution may be to reinstall the last good version of macOS. In this case, a week after releasing the bad update Apple issued new ones which are installed through Software Update as usual, and will put things right.