Cannot re-install OS on external SSD.

[KenSM]

I have a 2015 iMac 27” currently running Monterey, and I wanted to create a startup disk for it on a Samsung T5. I’ve done this before for various machines, including this iMac, and not had any problems. I first tried Command-R, and got to the point where the box says to select the disk you want to write the OS to – except that it doesn’t show any drives, not even Mackintosh HD. Next I tried Option-Command-R and this was even less successful – this time I got the exclamation mark and the 5101F message. When I try Shift-Option-Command-R I get the same result as with Command-R – ie no drives are shown.
Can anyone help?

 

[Fishrrman]

The 2015 iMac will currently boot on its own (without the external), is this correct?
What version of the OS is currently installed on it?

If the iMac boots on its own, do this (PRINT OUT these instructions):
Boot the iMac
Connect the t5
Open disk utility
Go to the "view" menu and choose "show all devices" (VERY important step, do not omit!)
Now locate (in the "list on the left") the line that represents the physical t5 SSD.
Click on it and click "erase"
Choose "APFS, GUID partition format"
Quit disk utility.
The t5 should now be "empty" and awaiting the OS install.

Now, power down the iMac -- all the way off.
Leave the t5 connected.
Hold down Command-OPTON-R
And KEEP HOLDING THEM DOWN.
Press the power on button.
KEEP HOLDING DOWN those keys!

Ethernet is best for this, but if you're using wifi, you'll need your wifi password -- enter it when prompted. (you don't have to hold down the key combo at this point)
The internet utilities take a while to load, be patient.

When the utilities are loaded, open the OS installer.
Start clicking through.
IMPORTANT: when the installer asks you WHERE you want to install, "point the way" to the external t5.

During installation, the Mac will reboot one or more times, and the screen will go dark for a minute or more with no other indication of activity. Be patient.

When done, you should see the initial setup screen (choose your language).
Start clicking through.

NOW YOU HAVE A DECISION TO MAKE:
From WHAT do you wish to install?
- a backup drive
- the iMac's internal drive, or...
- completely fresh?

If you just wish to "bring over" your apps and personal data from the iMac's internal drive, choose that when setup assistant asks if you wish to migrate from another drive.

Good luck.

 

[KenSM]

The 2015 iMac will currently boot on its own (without the external), is this correct?
What version of the OS is currently installed on it?

If the iMac boots on its own, do this (PRINT OUT these instructions):
Boot the iMac
Connect the t5
Open disk utility
Go to the "view" menu and choose "show all devices" (VERY important step, do not omit!)
Now locate (in the "list on the left") the line that represents the physical t5 SSD.
Click on it and click "erase"
Choose "APFS, GUID partition format"
Quit disk utility.
The t5 should now be "empty" and awaiting the OS install.

Now, power down the iMac -- all the way off.
Leave the t5 connected.
Hold down Command-OPTON-R
And KEEP HOLDING THEM DOWN.
Press the power on button.
KEEP HOLDING DOWN those keys!

Ethernet is best for this, but if you're using wifi, you'll need your wifi password -- enter it when prompted. (you don't have to hold down the key combo at this point)
The internet utilities take a while to load, be patient.

When the utilities are loaded, open the OS installer.
Start clicking through.
IMPORTANT: when the installer asks you WHERE you want to install, "point the way" to the external t5.

During installation, the Mac will reboot one or more times, and the screen will go dark for a minute or more with no other indication of activity. Be patient.

When done, you should see the initial setup screen (choose your language).
Start clicking through.

NOW YOU HAVE A DECISION TO MAKE:
From WHAT do you wish to install?
- a backup drive
- the iMac's internal drive, or...
- completely fresh?

If you just wish to "bring over" your apps and personal data from the iMac's internal drive, choose that when setup assistant asks if you wish to migrate from another drive.

Good luck.

Hello Fishrrman, and thank you for your advice. Yes, the iMac currently boots normally without the external, and otherwise it seems to be totally usable for everything I do on it. The current OS is Monterey, version 12.6.8. and the onboard storage is the original Apple 500GB SSD. Until fairly recently I had kept it on Catalina to preserve the ability to print from my old Brother printer and Epson scanner, but I upgraded when it became apparent that these could be made to work with Monterey.
After Command-Option-R I get the spinning globe with a progress bar beneath it, and after a few minutes I get the exclamation mark superimposed on the globe, with the wording apple.com/support beneath it, and the code -5101F. At this point nothing operates – not the wired mouse, not the wired keyboard. And yes, I’m connected by ethernet cable to avoid having to input the wireless password at various points. I escape from this by holding the power button for a few seconds, and when I press the power button again it boots normally.
Fortunately I have a number of more modern Macs so I’m not dependent on getting this working from an alternative drive, but it just nags at me that I can’t understand why it doesn’t work, when I’ve done this before on this machine and others, plus I wonder if when the time comes to give it away I may not be able to wipe the drive and restore a fresh operating system

 

[BrianBaughn]

Have you tried downloading and running the "Install macOS Monterey" application…choosing the external as the destination?

 

[KenSM]

Have you tried downloading and running the "Install macOS Monterey" application…choosing the external as the destination?

Good idea, Brian. I'm downloading it now, and will report what happens.

 

[KenSM]

Many thanks to both of you, Fishrrman and BrianBaughn, for your help. Downloading the Install application did the trick, and I can now run on the external.