External Samsung 980 PRO NVMe drive on a new 24” iMac

personally I would find 256GB a bit on the low side as by default the image lib and music lib and apps and home dir will be placed on the main harddisk and you need to take steps to move them

like

Move your Photos library to save space on your Mac – Apple Support (UK)

and Change where your music files are stored on Mac - Apple Support (IE)

and so forth

That was exact my setup for my "desktop" M1 -- however, if you to achieve better speed, you can choose more expensive Thunderbolt enclosure for the external SSD.

For the portable M1 devices (laptops) - I did order larger internal SSD capacity.

Stay with a larger internal SSD. SSD can last a very long time. I have a Mac that has an SSD for the last 10 years and no issue. Makes using the Mac much easier and efficient in my opinion as many have issues with external drives staying connected, etc.

I agree with tbirdvet in getting at least a 512 GB internal drive, larger if possible.

As far as external SSDs I heartily recommend those from OWC (MacSales.com). They are considered by most here to be the premier 3rd party supplier of Mac hardware. I have 7 external SSDs from them in a variety of enclosures and adaptors.

Do you have an external drive for Time Machine? If not get a rotational drive from OWC as they are very reasonably priced from large sizes and very reliable.

I would get the iMac with 512GB+. Always best to use the internal drive. I have an iMac that size and then I added an external NVME drive as an additional boot drive for testing new OS changes before updating my internal drive and another external NVME drive that keeps some of my home made videos that take up lots of storage space.

That’s correct. External SSDs, IMO, are more heavy marketing than engineering. They’re good for replacing internal mechanical drives, but other than that, not for everyone. As for larger internal SSDs, they’re not all gain and no pain - if they go south, which they could if used heavily, they’re more trouble to fix. Proceed with care, I’d stick to 256.

No, SSDs are more prone to failure if subject to intensive writes. That’s why one doesn’t perform defrags on them. Don’t try it…

Yes, definitely. Save more $ by skipping SSD and opting for regular USB 3. Copy files to the internal when in use and copy them back when done. A non-SSD external drive lasts longer, is more reliable.

well you can mount as volume as any directory so you can make say the download folder in your user directory really be an external harddrive mounted as that dir

Mount an external drive to a specific dir… - Apple Community

Mileage varies 😁 SSD Failure on 2 yr old Macbook Pro - Apple Community.

Ultimately, it all depends on how intensive the use is - if it’s heavy i.e. large file reads/writes, the likelihood of failure is high.

the reason one defrag magnetic rotating harddisk is the moving parts, mean if the sectors are not close, then the platter needs to in the worst case make a full rotation and move the reader from its outer position to its inner causing latency

there are no moving parts in ssd so no read / write latency Want to Defrag Your SSD? Here’s Why You Shouldn’t - DriveSavers (drivesaversdatarecovery.com) so one needs not defrag them ever

what you are thinking about is on a ssd each cell have a limited amount of writes before they die like a dvd-rw but more writes, but they work around the limited writes by using TRIM Trim (computing) - Wikipedia

Moving files back and forth to external is too slow unless you use an SSD as your external drive (depending on file size and how often you do this of course).

It depends on the user & application of course. I don’t use it at all! 256 + iCloud + external non-SSD plain vanilla USB works perfectly for my needs, thank you very much 😉🍻

Yes, I guess that applies to almost any hardware.