From Mojave in System Preferences / Software Update, I'm trying to update to Catalina.
Catalina takes about 8 GB.
The network cable was a bit loose, and after about 6 GB (75%), someone touched the cable and the download was interrupted with an error message: "Network connection was lost!".
I just clicked "Ok" (no reboot...) and was offered the update again. That I did. But the updater started almost from the beginning! (from 512 MB to be precise!). I was expecting MacOS to use the part that was already downloaded (or at least most of it), in order to spare another 30 minutes download!
Is there a way (update from command line...) to tell MacOS to consider using the part of the file that was already downloaded at the time of the disconnection?