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Historically I am sure older versions of OS X have sometimes forced me to install apps/files onto the volume/drive the OS is installed on.

Maybe I misremember that but regardless in current versions macOS Catalina onwards, do I have total freedom?

I am planning to get a new Intel-based Mac mini/iMac with the smallest SSD (256GB) and separately buy a large external SSD drive (2TB). I don't want to find that the internal drive is a limiting factor later - I might want to use the internal drive purely for the OS for instance.

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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.

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  • I don't currently use Time Machine but I plan to (maybe on NAS). I assume this can handle multiple volumes?
    – Mr. Boy
    Jul 22, 2021 at 11:12
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    Yes, TM will backup every connected disk, if you want.
    – benwiggy
    Jul 22, 2021 at 11:13
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    "You are unlikely to fill a 256 GB drive just with System and Apps." - this is a good point. It is comparatively small but still a massive amount of space. Many people mange fine with just that :)
    – Mr. Boy
    Jul 22, 2021 at 11:16
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    Without any attempt to constrain my boot drive's data, I have over 300GB on it already. It doesn't contain any of my 'heavy data' just 'regular stuff'. All my projects - music, photographs, VMs, etc - are on other drives, totalling 14TB at last count… & that doesn't include videos, which take up 12TB on another Mac. In short, if you have space, you will fill it. Even my partner, who uses a lot less space than I do, had managed to fill her 512GB drive, so when we got her a new iMac recently, I insisted she got a 1TB drive in it.
    – Tetsujin
    Jul 23, 2021 at 6:59
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    "Applications themselves can also be launched from any location." That, very sadly, is not entirely true any more. If your app is making use of a System Extension it has to be install in "an appropriate Applications directory of the system (read: /Applications).
    – DarkDust
    Jul 23, 2021 at 12:06
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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.

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    Parkinson's Law: "Stuff expands to fill the space available."
    – benwiggy
    Jul 22, 2021 at 11:27
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    I don't get how people can go through terabytes of storage on a computer... I have never exceeded 250GB on a 512GB MacBook Pro from 2013 (Big Sur running smooth to this day). Same with my iPhone XS, I've never exceeded 50GB/64GB phone. Why do people save so much (especially photos)? I take the Marie Kondo approach to life :)
    – AVelj
    Jul 22, 2021 at 14:05
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    @AVelj: An hour of uncompressed 4K video is 4 Petabyte. Now, if you are working on a typical 90 minutes long movie, you will actually have many different takes of the same scene, sometimes with multiple cameras. You can easily end up with dozens of Petabytes worth of video data. (Which is why this example is exaggerated and despite the loss in quality, video editing is often done in compressed formats.) Back when I still made music, I typically had about 500 GB of recordings of gigs and rehearsals with my bands on my laptop. I would over time review them, mix the ones I wanted to keep …
    – Jörg W Mittag
    Jul 22, 2021 at 19:11
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    @AVelj It all depends on what you’re doing. For example, it’s trivial to accidentally use up 1TB of space if you’re a gamer, because the norm for big name video games these days is at least double digit GB of space on disk. Same if you’re working with a lot of VMs and not sanely consolidating shared data and compacting disk images.
    – Austin Hemmelgarn
    Jul 22, 2021 at 21:00
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    @AVelj I have around 100k photos, taking up around 400GB. Part of the reason I have so many now is because it would be many days work to deal with it… it’s cheaper just to pay for the storage!
    – Tim
    Jul 23, 2021 at 7:37
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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.)

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I think you will find it makes things more complex with a smaller drive.

First the external drive will be slower. For compiling and media work that might well matter.

macOS defaults to having user directories on the boot container. This includes ~/Library/Application Support which can get big - in my case Steam is biggest here.

You can make links to put sonme things on external disks but that gets complex Some Applications check or otherwise have to be installed in /Applications (They are arguably badly written but many exist)

You will end up costing more in time than chosing such a small SSD. Especially if you consider what happens over time say in 3 years things will need more space.

I think you are better off using the money for the external SSD to buy a larger internal one now and later if you need more space buy an external one - You probably also want an external HD for backups, and that would also be fast enogh for storing media for playing

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    USB3/Thunderbolt allows for ~500MB/s. This is not going to be a bottleneck, the days of external drives being much slower than internal are behind us...
    – Mr. Boy
    Jul 22, 2021 at 10:46
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    I suppose I could use the external drive AS the boot volume?
    – Mr. Boy
    Jul 22, 2021 at 10:47
  • Not on Apple Silicon I think - and it will be slower
    – mmmmmm
    Jul 22, 2021 at 10:59
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    Apple also has ridiculous pricing for their SSDs. you can get a 2TB 970 EVO for $300 new and $200 used, and an enclosure for $15 to $30. Apple's 2TB ssd is $800. And external booting means you can upgrade to 4tb down the line by buying a new drive.
    – anonymousaga
    Jul 22, 2021 at 15:45
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    @anonymous Apple's "add-on" prices for RAM and storage are pretty consistent with those of Dell, HP and other manufacturers. There's more to the value than just a component that plugs in externally.
    – benwiggy
    Jul 23, 2021 at 7:18
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I'd recommend you boot right off of the 2tb SSD. I boot off of a 970 EVO in a USB 10 Gbps enclosure on my iMac 2017. It's blazingly fast and works without any hiccups. In fact, I've been using this setup for around 3 months, and I used to use an 860 Evo for a couple of years before that. It really simplifies your plan for saving space. If your'e into video editing and such, Id recommending using the internal drive for those files that benefit from sequential speeds. Also, I get 900 MB/s on my boot drive. I have heard of some issues with booting off external in big sur 11.1 and 11.2, but I think it's gone now

You'd need a 3tb external HDD, too. Its not strictly necessary, but time machine has saved at least some of my files around 30 times.

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  • I was wondering that. Storage is so cheap (bought not from Apple anyway) that 256Gb can almost be considered inconsequential, if booting from an external drive makes everything simpler.
    – Mr. Boy
    Jul 22, 2021 at 15:52
  • I’ve been doing it 3 years straight, with no issues. If you want to go nvme then I’d recommend amazon.com/dp/B08G14NBCS/… with a 960 or 970 evo Samsung ssd (I personally use a 970 evo because I got a 1tb for $70 used). If you want to go for Santa (500 mbps, perfectly usable, I upgraded for capacity and happened to switch to nvme), then amazon.com/dp/B011M8YACM/… with an 860 or 870 evo (or qvo) works well
    – anonymousaga
    Jul 22, 2021 at 19:15
  • It also removes worry if your buying used devices. For example, I don’t care about the storage for used iMacs since I boot externally. I also wouldn’t recommend Samsung’s pro series since it’s overly expensive for features (like pcie gen4) that aren’t important in this use case.
    – anonymousaga
    Jul 22, 2021 at 19:19
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    It's not going to be as fast as the on-chip storage in an M1, though; and the performance of swapping on the 8Gb RAM units is dependent on using the internal.
    – benwiggy
    Jul 23, 2021 at 7:15
  • @Mr.Boy so do you plan to boot externally? And you definitely need TM backups for this but you could try to attempt to make a Fusion Drive (if that command still works on m1) between the internal and external drive, though I wouldn’t recommend it because it would be unreliable.
    – anonymousaga
    Jul 28, 2021 at 14:40

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