Booting from external SSD Mac Mini 4

Your process is correct. After you have verified that your external drive boots correctly you could then erase all the duplicate folders from your internal. I usually just leave the internal the way it was when I made the external boot drive just in case I want to use that at a later date. If your external is the size you want there is no real reason you have to remove folders from your internal to make more room.

You have been given excellent advice.

Do not erase the internal SSD.

Simply get rid of any large files like collections of photos and videos and any third party apps you will no longer need.

If you ever require Apple Pay or AI you will have to boot into the internal.

What kind of SSD are you using?

NVMe in a Thunderbolt enclosure will give the best speed though even with a USB-C enclosure you are unlikely to notice any difference in the computer's performance.

My first attempts at booting from an external were with a SATA type SSD in a USB 3.0 enclosure and in timed tests the penalty was less than 5% when rendering or exporting videos from Final Cut Pro.

I later started using an NVMe in a Thunderbolt enclosure (far more expensive) to see what I was missing and the answer was "not much". Copying massive files was quicker but the ordinary handling was pretty much identical.

The only thing missing was the cash in my bank account!

+1

I would not wipe and take the internal back to its original state.

If the Internal is full, free up some space and keep it around incase there is a problem with the external startup drive.