why won't 10.15.3 install?

This is actually a really helpful suggestion.

It 'looks like' the online updater in the MacOS is not reporting errors correctly - so it fails and keeps failing without reporting the error. Hence the cyclic update - restart - update - etc.

So I downloaded the 10.15.3 update combo pack as recommended.

When run (in my case) - it failed - but reported an error.

This seems to be a big update, or certainly the combo pack is and it was failing on my laptop with "Insufficient Disk Space" error.

When I cleared space - it took quite a while to run and I think download some additional files.

BUT - the update completed successfully after the restart!

Thanks for the suggestion!

This update to 10.15.3 requires at least 12.5 GB of available storage, so if you don't have that you'll get stuck in the circle of restart-update-restart-update. I had this issue and once I moved things to an external drive and had enough space, it worked. You can view your available storage from the apple logo top left, system preferences storage.

Then you should offload files to an external HD and are up enough room to install and run properly. It's generally recommended to maintain a minimum ofd 15 GB of free space, more if possible, for optimal system and application performance.

This update will not warn you that you do not have enough physical space on the system. It will keep in an endless loop. Free yourself up about 10GB of storage space, then try again. You can use the link posted by Old Toad on page 1 of this discussion. That is what worked for me. I had to eliminate some junk from my hard drive before it could install.

Download and apply the macOS Catalina 10.15.3 Combo Update.

I had the same issue with only 10GB left on my hard drive. I freed some space and the update worked!

It wouldn’t work for me until I had 20 GB of free space. It’s recommended to have 12.5 GB, but I cleared up 15, and it still wasn’t working. But it did when I bumped it up to 20.

It may be possible that your Mac's internal drive may have some corrupted files. I suggest booting up in Recovery Mode, and then, use Disk Utility's First Aid to perform any needed repairs ... and then, try the upgrade again.

I had the same problem with 10.15.3 as I did with 10.15.2. For some ridiculous reason which makes absolutely no sense to me at all, the update will only install properly if I download it over an iOS device acting as a hotspot. I have a 2018 Mac Mini, I've seen iMac Pro users report the exact same problem and solution. I think the T2 chip is to blame.

macOS Catalina - Technical Specifications

General Requirements

• OS X 10.9 or later
• 4GB of memory
• 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
• Some features require an Apple ID; terms apply.
• Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013 or newer)

https://support.apple.com/en-us/HT210190

How to upgrade to macOS Catalina – Apple Support

And caution on 10.15, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

Try installing the update in Safe Mode with 20-30GB of free space...

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

After struggling like all of you on this issue, I managed to setup the update. It seems like you need more than 17Go available on your hard drive prior to the setup.

Good work, Apple should be clearer, you should keep 20-30 GB of Free space minimum because it isn't Free to us, theOS needs free space to even empty the trash!

Maybe you don't have enough free disk space, minimum 7GB for the update

Try installing from the Safe Mode.

Storage space, this update needed 9 GB of room on your disk