How to disable MacOS 10.14

[holty]

I have an elderly friend whom I support for their iMac issues. Recently I cloned their boot drive to an external SATA SSD to give the unit some much needed speed and hopefully longer life. All went well, however I am getting a number of callbacks as the owner is repeatedly booting into the wrong drive. How can I disable the internal drive, short of removeing it to stop these unnecessary callouts.I was reluctent to remove all the data as he was unsure of what he wishes to keep at this point in time.Nutting just the OS sounds the way for me however I have never disabled an OS purposely before.
Thanks in advance, Regards, John.

 

[TheIntruder]

Did you use the Startup Disk pref panel and set it to boot from the external drive?

There are Terminal commands that will also prevent a volume from auto mounting, but I'll allow others to address that.

 

[holty]

Hi CORP, yes I did and mentioned that they could tell if they weren't booted into the correct disc by the telltale icons within the boot system folders. Thank you for your response

 

[mdgm]

Which model iMac is this?

What interface is being used for the external SSD? Thunderbolt? USB-C? USB3?

 

[holty]

Hi magma, the unit is a late 2015 27inch 5K iMac iMac 17,1
The external interface being used USB3

 

[mdgm]

I can understand why you wouldn't have opened up the computer to replace the internal drive.

I think certain resets (can't remember which out of PRAM and SMC) would get the system to boot off the internal. Also if they happen to dislodge the external for some reason that could also reset it back to booting off the internal or the system not finding the external quickly enough for some reason.

I suppose you could setup a script to automatically change the boot disk and reboot, but that would require SIP being disabled which also would get reversed by a PRAM reset, I think.

The best solution would be to replace the internal drive, but also a difficult one.

 

[Nguyen Duc Hieu]

Hi magma, the unit is a late 2015 27inch 5K iMac iMac 17,1
The external interface being used USB3

Simple solution but sound scary to some.

Just format the internal HDD, and use it for data storage, or move all data to the external SSD, plus a copy on iCloud.
You can always re-install the OS to the Internal HDD, when you want to.

If you still want to use the internal HDD for data storage (this will create issues in the future, when OS on external and data on internal HDD)
- Add a volume to the Internal HDD from Disk Utility.
- Move all data to the newly created volume. Make links to point
- Erase the old volume.