"The connection to service named com.apple.lsd.modifydb was invalidated"

So it took me a while but I did everything in the list you recommended. The problem with the computer crashing shortly after rebooting each time I believe has resolved itself, however when I check Activity Monitor, 'secd' still occupies over 123% CPU usage for about the first five minutes after reboot, but then it goes away. I hope that this is okay?

The one big issue I still face is that I can't get my Mac to update past 10.13.4. I have tried multiple times to update to the High Sierra 10.13.6 update, but each time after going through the motions of downloading and restarting, it just doesn't work.

Another issue is that my Time Machine icon doesn't always appear on my desktop after rebooting sometimes. This is an internal drive so I'm not sure why the computer doesn't recognize it every time, even though it has appeared and worked to backup in the past couple of days. It just seems to be completely random whether it appears or not - before all these recent issues, I had absolutely no problems with it.

I'm also curious, since I've now deleted all of my antivirus software from my Mac, what should I be using to protect my computer? Is there one actually recommended out there in the Apple community?

Thanks again for all of your help!

So I tried the 10.13.6 Combo update from Apple, but after going through the motions, still no luck to change from 10.13.4.

Also, before I did this, my Time Machine was back on the desktop, and it successfully did a backup, but after attempting the update and rebooting, it disappeared again! One final issue, in Activity Monitor, after about 5 minutes when 'secd' finally disappears, my CPU is now being taken up by 'safaribookmarkssyncagent'. One problem for another! :/

I read through your own troubles of trying to update through the link you sent me, but I'm kinda at a loss as to exactly what you did to finally get it to work....would you be able to repeat those steps here, for myself and for anyone else who might be having the same trouble?

Also, any idea on who to get my Time Machine to stay on my desktop, and how to get rid of that annoying 'safaribookmarkssyncagent' high cpu usage bug?

Thanks so much for your assistance!!

Sorry for the delay...

I don't really use Safari, only Firefox, so it's not open, and even though I didn't knowingly set up iCloud, when I went in into System Preferences to uncheck the box to turn it off (after finding out how to do that!), it seemed to have maxed out at the 5GB that Apple gives you. I turned it off, and still have SafariBooksSyncAgent at the top of my Activity Monitor while in Safe Mode. Diagnosticd, Windowserver, and Secd were also above 100% after boot up, but eventually went away after about 10 minutes. Now I'm left with the 'Safari...' message still at the top with just under 100% usage.

Time Machine did come back on my desktop in Safe Boot though, and actively went into backing up right away.

Yes I do have an external drive that I can use.

Thanks again.

Follow up...

I booted back into regular mode, and boom, SafariBooksSyncAgent was gone from Activity Monitor! I guess signing out of the iCloud that I didn't know I even had solved that one. My Time Machine was also back on my desktop, but as I said before, it's been kind of random as to whether it appears or not, so I just might have been lucky this time?

'Secd' is still at the top of Activity Monitor right after boot up though, with the same "The connection to service named com.apple.lsd.modifydb was invalidated" message over and over again in Console, until it finally goes away after about five minutes.

According to the instructions, I need to begin the process to enable iCloud keychain. Is this done by going into Preferences>iCloud, then clicking on the checkmark beside Keychain? Mine already had a check beside it, and when I went to uncheck it, it said if I did it that it would delete all passwords and credit card info from my computer, which I don't really want to do as this would remove the autofill on sites that I need it. Is there perhaps another way of accomplishing this? As I mentioned before, the 'secd' seems to go away on it's own now after about 5 minutes, so I guess it's not causing too much trouble anyways?

Just wondering though about unenabling iCloud altogether, as I did the other day to get rid of the 'SafariBooksSyncAgent' issue. Isn't this only a temporary measure? Does that mean if I actually wanted to use iCloud then I'd be stuck with that problem? There must be a more permanent solution?

However, I'm still unable to update to 10.13.6.... are you able to list the procedure to enable this?

Thanks!