Mac Mini M4 will not boot from external HD

There is no real problem as long as you format the external drive the same as the internal one and install Sequoia on it (I use an Orico 2Tb SSD in an USB-c Orico enclosure with heat sink and fan – probably necessary). I found it better to transfer manually my contents and apps rather than use the legacy option. The only thing is that Time Machine won't work with the external drive – I spent a frustrating 3 1/2 hours on the phone with Apple technicians before giving up the ghost. The way I found around that is that before going to bed, I restart from the internal drive with the TM option set to "Save once a day". When I get up in the morning, I restart from the external drive... and the Time Machine has done its job, the new restoration perfectly accessible from the external drive. Not such a big price to pay for a much faster and roomier system disk.

There could be several reasons why this problem exists but here is what I would do.

• Reformat the SSD only using Disk Utility. Do not use any so-called maintenance apps which may have come with the SSD. They do cause problems. Make sure the SSD is formatted the same as your Mac's HD is.
• Download and reinstall the OS. The original OS on it my have been corrupted. This will often happen when it is downloaded via Wi-Fi. Better to use a wired Ethernet connection. If that is not possible, then you may have to do multiple downloads to get one that works.
• If the problem still persists, I would replace the SSD.

When you format the SSD make sure you format the Physical Disk and not a Volume.

I made that mistake a year ago and wondered why it would not boot.

I use WD either Blue or Black series NVME. The fastest is the WD SN850X in my Acasis TB enclosure

I did not change any security policy.

I’ve been fighting the same issue all day and realised I have a crucial ssd.. like you say - worked fine with my m2 so thought that wouldn’t be an issue. Ball ache

I always use Option 1 . . . it's the quickest, simplest and safest.

"Silicon CPU" ? They're all made of silicon! The options are Intel or Apple sources.

Follow Matti's first suggestion . . . download the macOS installer and install it on the external after you have formatted the PHYSICAL DISK (not volume!) as APFS.

You should have no problem.

Thank you very much !

Apologies --

sudo diskutil partitionDisk drive(xx) 1 gpt jhfs+ DriveName 100%

should read

sudo diskutil partitionDisk drive(xx) 1 gpt apfs DriveName 100%