2014 iMac won't restart or shut down after external SSD installation

I'm trying to get my daughters 2014 27" iMac up and running after the internal hard drive failed (flashing question mark)

I thought instead of taking the screen off and installing a new drive i could run it of an external drive, so managed to install Os and all apps seem to work ok but the trouble I'm having is that it won't restart or shutdown i have to switch it off manually then the reboot take an age sometimes i have to hold down the option key and pick the drive to boot from, it only shows the new ssd.Is the mac looking for the failed drive first.

When i click restart all the open windows vanish also the menu bar and it sits there for at least 5 mins with the desktop picture and the dock then the screen goes blank and a message says "the mac restarted because of a problem press any key" then i'm back on the login screen.

If i click shutdown the screen goes that dark blue and sits there for about 5 mins then restarts to the login screen.

this is a brand new ssd with a clean install i started with Yosemite to High Sierra to Big Sur once up and running everything is working fast and clean

[Re-Titled by Moderator]

Original Title: Restart and shutdown problems