Is combining internal and external SSDs on Mac mini M4 possible?

Yeah' Apple really needs to increase the internal storage in the base model Mac mini to at least 512GB.

If you have had that one for less than 14 days, consider returning it and ordering one with 512GB or 1T of storage.

Combining as in RAID 0 or by Fusing, is not a very good idea.

Because if one drive has an issue or fails, then you lose everything.

One option is to keep macOS and App's on the smaller internal SSD.

Then move and keep your Photos, Music and Movie libraries on the external SSD.

see > https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

Another option is to install macOS on the external SSD, then startup and run from it.

see > Use an external SSD as your startup disk … - Apple Community

Either way, in addition to using that 1T external SSD.

You really need to have a second external HDD or SSD for backup.

for example:

Those can be Time Machine backups on an external HDD.

see > Back up your Mac with Time Machine - Apple Support

Or they can be clones using CarbonCopyCloner or SuperDuper on a second SSD.

https://bombich.com/ or https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

FWIW, even if it was possible, a failure on the external would render the whole system useless and you would need todo a recovery procedure on the Mac to bring it back to life.

It's best to take one of two approaches:

1. Put all data possible on an external drive (which is pretty much everything) and only keep apps on the internal drive. Actually you would want two externals, one as the daily use and on as a backup.
2. Crete an external boot drive and operate from that. One thing to note however is that many features will not work on an external (Apple Intelligence, Wallet, etc.).

I personally use #1. Since I have multiple Macs I can simply move my "data drive" from one to another. Also, should anything go wrong and you need to get a new Mac or simply need to wipe and reinstall the current one, you would only need to restore your applications then just plug the "data drive" in. You will need to direct things like Music, Photos, etc. to the location on the external but that is a very simple task.

Why would you want to combine them? It's just as easy to use them as separate entities.

Regarding your choice of a 256GB drive you should have no problems. I have bought the base model M2 and M4 and simply use plugin USB-C or Thunderbolt 1TB and 2TB NVMe drives.

I have taken it a stage further and actually boot from the externals, with no loss of performance, which leaves the internal untouched unless you deliberately choose to store data on it.

The advice to buy a Mac with a larger SSD is fine for those who don't mind being ripped off . . . the extra 256GB you get by opting for the 512GB Mac will cost £200 whereas a 2TB plugin NVMe will only set you back £100 leaving you with plenty of cash in hand for something more useful.

Of course if you really wanted to spend £200 in the first place you could get a 4TB NVMe!

Sorry when I edited, some forum glitch changed that first link to the SuperDuper link.

It should be:

One option is to keep macOS and App's on the smaller internal SSD.

Then move and keep your Photos, Music and Movie libraries on the external SSD.

see > Free up storage space on Mac - Apple Support

This kind of sounds like the user whats a Rolls Royce but can afford to pay for the Fuel

And now they are looking for a way of the short tightness on the original purchase

FYI, I have an older M2 Mac Mini with the 256 GB SSD / 8 GB Unified Ram. It is used primarily to Live Stream show on the TV in the Livingroom. No issue with that usage

Also have the M4 Mac Mini with the 512 GB SSD and 16 GB Unified RAM for most other Non Streaming purposes . No issue there too

So the point being in your case.

This will all depend on the your current needs are and what your further needs may become

Each of my follow contributors make some good points