I just got a 2TB external SSD, primarily to download, and run, my Photos library on. I am now also considering installing macOS on it, as I have discovered a few applications whose installers take up considerably more space than is currently available on my M1 Mac mini's HD.
The last time I partitioned a drive was several years ago, to have the option to boot from different (Intel) Mac OS versions, so my memory is kind of hazy on what the criteria are for partitioning. My question is: Would it be advisable to create a partition to install Ventura or Sonoma on, seeing as I will want to have other file types hanging around on the new drive?
I already formatted the drive as Mac OS Extended (Journaled), but it seems that if it was formatted as APFS then partitioning is not relevant anyway?
I normally partition my system/internal disk for my computers as well as family whose computers I assist. Additionally I sometimes partition external disks depending on what it is for/how it will be used.
I've been partitioning my system disks since my PowerBook G4/MacOS X 10.1 days which itself is probably a holdover from UNIX system administration days. Some of the reasons no longer apply but I still have my reasons.
Then you're correct that if you format a drive APFS then you can create multiple volumes on it without creating multiple partitions and it has the advantage that those volumes can all share the free space, avoiding the need to manage partition sizes and try to grow and shrink partitions. However, the volumes in an APFS container must all be APFS so you still need partitions if you need multiple filesystem types (e.g. one APFS and one exFAT). Additionally, partitions force separation. I don't know what new versions of MacOS will do to my system volume and the data on it during upgrades but I assume it will leave other partitions alone.
In any case, for any recent version of MacOS, you'll be using APFS for the system and it is generally recommended to use APFS over HFS+ on all flash/SSD storage. Although unlikely relevant in your case, spinning hard drives should not use APFS and so stick with HFS+ for Mac usage. For external drives that you might or plan to use to share data with non-Mac systems, exFAT will give you the best compatibility. For example, if you ever had to print something from a USB stick at a Fedex/Kinkos, those printer/copiers won't see an HFS+ or APFS partition last time I used one.
For the internal disk, I typically have 2-4 partitions: two to handle different OS versions, if a laptop then one to keep my home directory separate from a particular OS version, one for Bootcamp/etc.
For external disks (e.g. SSD), it depends. For simplicity's sake for an SSD that you will only use with Macs and won't be running the OS, one large partition as APFS should be fine. Then my Mac mini, I point my home directory to a folder on that drive so that I am not dependent on a drive soldered into the hardware. Plus I deliberately purchased the smallest internal drive available so that I could buy extra storage at market prices rather than Apple prices and replace that storage with a large drive anytime I wanted.
If your OS + applications + personal data is exceeding your internal drive, you could move your OS to an external drive as noted elsewhere but it's not ideal for reasons mentioned elsewhere. Among others you don't eliminate the dependency on the internal drive for system operation and performance is likely lower (though everything is so fast these days it may not be noticeable). On the flip side we've seen the continued shift towards applications that must install to /Applications (plus only installing as an administrator even though that's rarely necessary) which means your system drive/partition has to be pretty big, which is the problem I'm guessing you're having.
If your OS + applications is comfortably less than your internal drive (I'm guessing at least 256GB?), I would move your entire home folder, etc to the external drive before trying to run the OS + applications from an external. Also if you have applications that don't have to be in /Applications, you can keep those in your home directory (~/Applications), too. I haven't run into any problems keeping my home folder on an external disk while maintaining a symbolic link from /Users/login -> /Volumes/externaldrive/login (in theory you can just tell the system to maintain your account in a non-standard location in Users & Groups but I didn't find this reliable under much older OS and applications and haven't tried again recently). Assuming a 256GB internal drive and 60GB for macOS (base + room for system updates + room for bloat), are your applications (not counting their data) more than 150 GB?
