Installing and Running macOS on an External SSD

Please proceed at your own risk. Please note that I cannot take any responsibility for any issues that may arise from following this article. Please proceed with the understanding that various problems, such as data loss or hardware failure, may occur.

Reasons

Cost

1. Adding an extra 4TB SSD to a Mac costs about 180,000 yen more.
2. It seems unlikely that this pricing situation will change.
3. If I want to upgrade from a 4TB Mac to a new one, I would have to buy a 4TB model again. (I wanted to drop out of that rat race.)
   

Usage

1. I recently started composing music with Logic Pro and wanted to separate my environments.
2. If I place a Mac Studio in the office, I can carry my environment just by bringing the external SSD. In other words, it significantly reduces the amount of luggage.
3. It was more straightforward than moving only Xcode data to an external SSD for operation.
4. Building app data stored on an external SSD caused errors that I couldn't resolve.

Impressions

It feels good so far. I have no complaints regarding the performance.

However, since an SSD is sticking out of the Mac, I think it might be difficult to work with it on your lap while traveling. If the SSD gets disconnected, it results in a forced shutdown. It might not be suitable for people who carry their Mac around frequently.

What You Need

1. Thunderbolt-compatible SSD Case *PCIe tunneling required

Cable included & Cheap

https://amzn.to/4p5m6QN

Cable included & With cooling fan

https://amzn.to/4hLe7FM

Cable included & Fanless

https://amzn.to/4osIuDs

2. External SSD

1TB

https://amzn.to/47J9pUy

2TB

https://amzn.to/4r6R9NZ

4TB

https://amzn.to/49a7UB8

8TB

https://amzn.to/49IwV6F

3. Thunderbolt-compatible Cable

https://amzn.to/3JJEq2A

1m Cable

https://amzn.to/3WKQ9kw

How to Do It

Most of the instructions can be found here.
https://support.apple.com/ja-jp/111336

The following is written for my own reference as a memorandum.

1. Plug the external SSD into a port other than the DFU port.

https://support.apple.com/ja-jp/120694

2. Format the SSD

Format in APFS format
https://note.com/kunren/n/n4880d3136e0e

3. Download macOS

https://support.apple.com/ja-jp/101578

Since my Mac was running Tahoe 26.0.1, I downloaded Tahoe using the following command.

sudo /Applications/Install\ macOS\ Tahoe.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

4. Install

Please select the external SSD and install.

Be careful: if you select the internal storage at this point, your data will be wiped out. Please ensure you select the external SSD.

5. Boot

Shut down the Mac, then press and hold the power button to start. *Appears to be for M1 or later

Press and hold until "Loading startup options..." appears.

Once it appears, you can release it.

When the screen for selecting the startup disk appears like this, select the external SSD to boot.
In my case, from the next time onwards, it started booting from the external SSD automatically as long as the external SSD was plugged in.

Tweet from a Predecessor

I decided to try this after seeing this tweet.
https://x.com/mokkos_org/status/1920540148929839286