Here's a weird one:
I downloaded the Combo 10.14.3 Update and tried to install it, but got a message that my internal disk (not to mention the externals) did not meet the requirements for the update. I then went to Software Update and it said that my 10.14.2 system was up to date.
Upon restart, the Combo gave me the same message but SU was able to download and install the Delta update. After installation, I opened the Combo and was given the green light in case I wanted to install it over my 10.14.3 system. (I won't do that unless I notice some issues.)
Any ideas?