Are there any cases in modern macOS where you have to put files on the OS' volume?

Solution 1:

First of all: the largest contributor of 'stuff' to most people's disk storage is User documents.

None of these have to be in the user folder, and there is no problem opening and saving from an external drive (and there never has been). If space is a concern, then I would simply store all your documents on an external.

Don't forget that you'll need another external drive for your backup disk.

Most Applications can also be launched from any location. However, some applications install a variety of support files, which need to be in /Library/Application Support or other Library subfolder, (or the user's equivalent Library folder).

Apps also have working caches, preferences, and other files which are saved in the User's Library.

I would recommend leaving applications in their default location. You are unlikely to fill a 256 GB drive just with System and Apps.

Other files, like Photos.app's library, "iTunes" archive, Garageband samples, etc are normally stored on the system disk, but can be configured to other locations.

For a laptop, I would always recommend getting the largest internal storage you can afford (and then maybe some more!); but for an iMac or Mini, there's no inconvenience from an attached device.

Solution 2:

No. We have thousands of Macs at work with 256 storage as a standard. People that fill 256 also tend to fill 512 and tend to fill one TB boot drives.

Short of needing three or more OS for testing, it’s rare to need that much space for the OS. Spend more on storage due to convenience, not need is my advice.

Solution 3:

If an app is bundling a system extension, Apple requires you to start the app from an "appropriate Applications directory of the system", which usually means /Applications.

This is probably a requirement to make it easier for the OS to track the System Extension and its host app (like detecting and handling updates). Once the app has been started and the app has requested the installation of its System Extension, Apple actually copies it somewhere else (a subdirectory of /Library/SystemExtensions) and starts it from that location. (You can see this with ps aux if you know the name of System Extension; you can list them using systemextensionsctl list).

macOS is somewhat tracking the app's directory at this point. If you try to move or delete the app using Finder, you get this message:

The application “Foobar” is hosting system extensions. These extensions will be removed if you continue.

(Deleting/moving via the shell leaves the extension untouched. It continues to run; after all, it actually lives elsewhere.)