Using external drive as start up disk

Charles Tucker wrote:

I partitioned the new SSD into a 200GB "OS partition" and use the remainder as the "Data partition." I saw this suggested on another site so that clean installs of the OS could be done without losing any data.

I never recommend for users to have multiple partitions on their drives unless you know exactly what you are doing. Most users will choose the wrong sizes for their partitions and only realize it after one of the partitions is running out of room. It is a hassle to fix since it requires erasing the whole drive and starting over again which most people find annoying and don't want to do. Yes doing this can make it easier to perform a clean install, but most users never need to perform a clean install.

Instead just use a single partition and just make sure you have frequent & regular backups. If you ever need to perform a clean install, then afterwards you just use Setup Assistant to restore your apps & user accounts from the backup.

Why did you give the OS partition 200GB? macOS doesn't need anywhere near that much space even if you have a lot of third party apps installed. Most users need more storage space for their own personal files.

My question is, what should be kept on each partition? I have my applications in the Applications folder on the OS partition. Is that where they should be?

This is another reason why I discourage users from using multiple partitions as it makes things overly complicated. You should just install apps normally. All apps need to be installed to the main "Applications" folder. There may be some apps that can be installed to the local "Applications" folder within your home user folder if you do not want other users to access the app.

Since I copied the old HD to the Data partition, it has an old OS on it as well, but I wouldn't use it to start up. That partition has System and Library folders on them - are those used at all? Can I delete them?

You can delete all the old system files from your data partition since like you said they were copied from an old drive and are not needed.

You can do that all with a single partition. I actually did that on my last new Mac because the previous one had some issues which I didn't want transferred to a new system. I found that migrating just the user folder also made changes to other system areas for some reason. So I performed a clean install and manually transferred my data and certain preferences over to the new Mac.

FYI, beginning with macOS 10.15 Catalina macOS now automatically separates the macOS system files from the user data contained in the home User folders by using a separate APFS "volume". This is kind of similar to using multiple partitions, but it doesn't require assigning storage size limits.

https://support.apple.com/en-us/HT210650