Success!If there is nothing of value on the “new” Mac mini then I would second the nuke and reinstall method - or copy everything off, nuke and then copy back. If the OS is acting funny on thr “old” Mac a copy/clone might act funny on the “new” Mac.
To go the nuke and reinstall route, you’ll need a 16GB USB drive.
1. Download Sierra from here: https://support.apple.com/en-us/HT211683 and follow steps #1-3.
2. There is a “bug” in the Sierra Installer, so you need to follow the steps here from Eau Rouge: https://discussions.apple.com/thread/251380944 to create thr USB installer
If you need more info on thr createinstallmedia step, see here: https://support.apple.com/en-us/HT201372
3. When your ready to wipe the drive and reinstall, plug the bootable intaller into thr Mac, press and hold the option key, and then power on the Mac. Once the boot menu is shown you can release the option key. Select the usb drive to boot off of, load disk utility and erase the internal drive, then reinstall Sierra. Once complete perform the available updates and then reinstall your other apps and finally copy the data over.
I now have a USB stick that boots to the new mini.
I clicked on 'install macOS but it wants to install it on the USB stick. It's already on there! How do I get it to install it on the internal drive? Disk Utility? Erase the 1tb drive?
Thanks.