Delete old OSX install from HDD in old iMac

[charltonfan1]

My folks' 6 year old iMac was having issues with its HDD so I installed OSX latest version onto an external USB 3.0 drive and set it up to boot from that. It has been working for months fine like this

Sometimes recently however the machine boots to the old HDD OSX install so I want to delete this without formatting the whole drive. Or disable it from being visible to the machine

How best should I do this and as I can't visit to do myself please make it as simple as possible so my old man can manage the steps!

Thank you

 

[charltonfan1]

In the end i tried everything and the iMac was refusing to even recognise boot commands both trying the apple wireless keyboard and a USB one, rendering my attempts futile. Apple has obviously changed MacOS and how it sees external drives upon boot causing the underlying problem

They ordered a new Mac Mini and will restore a time machine backup of the USB HDD MacOS install when this is setup

Thank you anyway