Which Disk is the Target Disk?

tcarder wrote:

It is clear I am not explaining my situation well at all. All I am trying to do is to make my mini 2009 and mini 2014 accessible to the mini 2020.

There are a few ways to do that.

Target Disk Mode (Intel-based Macs)

One is to boot the 2009 Mac mini and 2014 Mac mini into a special mode where they act as external disk drives, and nothing else. That is called Target Disk Mode – and here, "Target" refers to the Mac running in the special mode, regardless of the direction in which you are copying files.

Transfer files between two Mac computers using target disk mode - Apple Support

Whether you can use this mode is a bit more complicated. A 2020 Mac mini would be an Apple Silicon Mac mini, running Big Sur or later. According to the Support article,

"If either of the computers has macOS 11 or later installed, you must connect the two computers using a Thunderbolt cable."

This would be impossible with the 2009 Mac mini – which does not support Thunderbolt. It would be possible to make a Thunderbolt connection to the 2014 Mac mini, but it has an older form of Thunderbolt, so you would need both a $50 Apple Thunderbolt 3-to-2 adapter and a $30+ Thunderbolt 1/2 cable. It would have been nice if Apple had kept support for USB Target Disk Mode in Big Sur and later – but they didn't.

"Target Disk Mode" (Apple-Silicon-based Mac)

You could set up your 2020 (M1) Mac mini to act as an external drive for one of your older Mac minis. In this case, the connection does not need to be a Thunderbolt one. It could be a Thunderbolt one, but a USB one would work just fine. You can find the instructions for this here:

Transfer files between a Mac with Apple silicon and another Mac - Apple Support

Unfortunately, this doesn't seem to match your use case (2020 Mac as "computer", 2009/2014 Macs as "disks").

File Sharing

You could set up your 2009 Mac mini and 2014 Mac mini to share some of the files on their disks over the network and then mount the network volumes on your 2020 Mac mini.

Set up file sharing on Mac - Apple Support

Connect your Mac to shared computers and servers - Apple Support

Other Options

If all of your computers are connected to iCloud under the same Apple ID, you could copy specific files which you want to share into iCloud Drive. iCloud Drive will keep changes to those files synchronized between all the Macs. (Note that you would not want to copy a database like the iTunes Library or Photos Library to iCloud Drive. There is a separate feature – iCloud Photos – that you can use to synchronize Photos libraries.)

If you just want to do a one-time transfer of files from the 2009 and 2014 Mac minis to the 2020 ones, you could copy files onto an external drive and transfer the external drive.

It seems to me that there might be some confusion with the expression "target disk" and, possibly, "target disk mode". The latter is meant for a specific situation where a mac is directly attached to another, and working just as a disk. This is NOT what you want if you are connecting all your macs to a local network.

Instead, what you want to do is Share the disk in some macs, and mount them in other macs (while all the macs are running normally).

If you can get a friend to help you locally, that will be far easier and less stressful than a back and forth here.

It takes just a minute to share a disk from one mac and access it from all others on the local network, but if you have never done it can be confusing.

Once you have them connected, drag-copy your documents and as many compatible applications as you need to the corresponding locations on the new computer. You can also transfer the files through the Migration Assistant, but it puts home folder contents into a different user account.

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You won't be able to boot the new computer from the 2009 mini, and will only be able to do that with the 2014 mini if it's running at least Big Sur; use an application compatible with 14.6.1 or newer to open your documents if possible.

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tcarder wrote:

Just one more issue: I will be able to set either of the external minis as the startup disk then restart to directly access the files on them under the MacOS resident on that external disk, right? That way I will be able to use the files on the older minis (the 2014 [MacOS 10.14 Mojave and the 2009 MacOS 10.11 El Capitan]) under the MacOS resident in the external disk selected as the startup disk, right?

No.

The Mac mini (M1, 2020) originally shipped with macOS 11.0 (Big Sur). As a rule, Macs cannot run any version of macOS earlier than the one originally shipped with their hardware model. That would rule out booting a M1 Mac mini off either a Mojave startup disk or an El Capitan startup disk even if all of your Macs used the same type of machine code.

The M1 Mac mini does not use the same basic type of machine code as Intel-based Macs, so you would not even be able to run El Capitan or Mojave on it by installing those OSes in a Parallels virtual machine.

Some of your old Intel applications may run on the M1 Mac mini – provided that they are 64-bit applications which do not rely on Intel-only system code (e.g., kernel extensions). This is thanks to the Rosetta 2 translator included with Apple Silicon versions of macOS. The first time you run an Intel application on an Apple Silicon Mac, there is a short delay while Rosetta 2 translates Intel machine code to Apple Silicon machine code and saves a copy of the translation. On the second and subsequent runs, there is no delay, because the Mac uses the saved translation.

A further question. Is your goal:

• To keep using the 2009 Mac mini and 2014 Mac mini after you have set up the M1 Mac mini – i.e., there is going to be an ongoing need for three computers to have access to the same files?

OR

• To transfer everything that you want to save (and can save) to the M1 Mac mini – so that you can retire the older machines, or use them for other things that will not involve ongoing sharing of files?

Whether your goal is file sharing or migration could make a difference in terms of the advice that people would give you.

The target disk is the one you're copying the files to; it's the target location for those files.

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