How to install OS 10.15 on External Drive?

Thank you for the info.

I have formatted the external drive as APFS and GUID.

But each time I try to download Catalina, the download goes through "almost" the complete process but then stops. An error message states, each time, that the process was corrupted and the download has been deleted.

I have tried this several times, and even tried to download a different OS version, but still have the same results.

Any suggestions?

Did not download.

I Restarted in Safe Mode (I assumed it was Safe Mode because I could not find a way to confirm this)

I used the link you provided and when I clicked on Catalina, the process open the App Store to the choice of Catalina. I selected "Get" and then selected "Download". The screen displayed a Process Bar for a few minutes and then displayed the following error again.

Installation failed

An error occurred while installing the selected updates.

I was never given a choice of selecting where to install the OS.

So I don't know if the process was trying to install on my primary drive or not.

Is there any other guidance you can provide?

NOTE:

I noticed that the webpage you directed me to provided the choice to ""download" older OS versions.

For versions 10.7 thru 10.11, I was able to download the Install Package as .dmg files.

But OS versions 10.12 and higher would Not download a .dmg.

Version 10.12 and higher always opened the App Store and went thru the process I described previously, which continued to fail as noted.

Thank you for taking the time to try and assist me.

I am using 10.15.7 on a 2012 Mac mini.

I am trying to set up the external 1 TB HDD with 10.15 to have as an alternate start-up drive in case the drive in my Mac mini should start acting up.

I have searched my Mac mini trying to find the Catalina (10.15) I must have used originally to upgrade my Mac mini.

But I have not been able to find it.

I have read that Command-R can be used to Reinstall macOS from the built-in recovery disk on your computer:

Do you know if this is an option for me, if I can't get the Install to download?

Thanks again for your time.

I used Command-R and a folder named "macOS Install Data" is now on the External Disk.

But after completing the process, an Error Message stated the the OS could not be installed on the drive.

I don't know if the "drive" referred to was my Primary Drive or the External Drive.

Should I try to launch one of the files (listed below) that were loaded on the External Drive?

If yes, what am I likely to "see" during the install on the External Drive?

I ask because all of the instructions I can find state that the install will put an Install App in the Applications Folder.

But I need the install to occur on the External Drive.

Thanks for your time and assistance.

I tried to use the "Partition" in Disk Utility as you noted.

I immediately received a message that the process Failed.

I tried this several times but it always Failed.

I was able to use the "Erase" in Disk Utility, selecting "APFS" format and "GUID Partition".

It completed the formatting successfully.

I then restarted using ⌘R after shutdown.

The install appears to have never completed.

After several hours the Process Bar was still sitting at the very beginning.

I shutdown the Mac mini and after restarting I found the same type of files as I noted previously.

So, I know the process should work but isn't for me.

I will try using a different external drive and see what happens.

Tangent question:

You said to use "APFS" with 10.10.15.

I noticed that the HDD I use for my Time Machine backups is formatted as "Mac OS Extended (Journaled)".

Should it be formatted as "APFS" or "APFS (Encrypted)"?

Also, do you know where I can find an explanation the the various formats and when to use them?

In example, what does "Case Sensitive" mean?

Thanks again.

I previously replaced the Mac mini's 1TB HDD with a 2 TB SSD.

The external drive I was trying to reinstall 10.15 on is the old 1 TB HDD, just to have on hand.

I can access the old 1 TB HDD (APFS / GUID) as an external drive and storage files on it.

So, I assume the HDD is "working".

But, if the old HDD is "going bad", would that prevent installing 10.15 but still allow files to be various added?

On another note, I get an error message when I connect my iPhone SE Gen 3 to the Mac mini to back it up.

The message states that the OS must be updated to connect / work (my words) to the iPhone SE.

But the Mac mini can't / won't complete "any" automatic or manual commands to updated to the OS on the Mac mini. If I ignore the message, I am able to connect / manage the iPhone SE, without any obvious issues.

Do you think these issues (can't load OS on the old HDD (external) & can't / won't update beyond 10.15.7) could be caused by something else in the Mac mini?

It is a 2012 and so is pretty old.

I am hoping to get another year out of the Mac mini.

I'm waiting to see if Apple offers a M2 (or M3) iMac in 2023.

Many thanks.

Than you again for all of your time and assistance.