Time Machine does not have enough room and other oddities

So this started when i tried to apply the Mojave Security update 2019-002 that has been hanging around in my Software update for a while, I have an iMac Pro. When I tried to run the update, it never finished and had a black screen for a long time. Overnight long. I restarted and tried again. same result. Tried to boot up in Recovery to run Disk Utility on the start up disk, but that didn’t work. Could not run in normally when started up. Would just get spinning beach ball after a while. I was able to log in each time. So I tried to download the update and apply it. That did not work.

Now I am getting concerned something is wrong and I thought that if I reinstalled the MacOS, it would clean things up and then apply the update. The OS installed OK but the update was still red. I decided to forget about it for a while and get on with life.

Then I got an error while the Time Machine was trying to back up, which I have been doing for a while. It said there was not enough space (I have a 2 TB SSD drive) with a 8 TB USB TM (and others). I thought that was weird and dismounted the TM and then added it back. Still would not back up.

I have enough mirror images and backups, so I formatted the drive and decided to start over. First it asked me if I wanted to back up an encrypted disk to a non encrypted TM. I checked, and my other backup were not encrypted and my FileVault was not on. I tried again and got this

Which is weird since my drive is only 2 TB. What are my options?

This does seem really weird. The main thing I’d suggest you try initially is to use Disk Utility to verify all the drives in play.

If that doesn’t resolve it, the best thing might be to call Apple Support.