backup disk on iMac

Hi; I call Apple Support yesterday afternoon. Support had me run a fix disk on my external hard drive- nothing was wrong with it. Then they had me disconnect the power button in the back of iMac so that

the computer could do a system reboot (a fancy correction to the computer itself); because it actually thought something was wrong with the hard drive (which we knew was not true).

Then I was able to disconnect my external hard drive.

Thanks for your help and insights. Reading the links has helped me learn more about the computer.

Karen

It is not the system restart option under the Apple icon on left side. It is actually disconnecting the power cord on the back of the monitor. This is something done by technicians (leave out for 10 sec) and then replug into back of computer. It causes the computer to go thru much more than just a reboot.

Thanks for your input on my original question.

Karen

Forgot to say using Catalina; can NOT remove (disconnect) the whole backup disk from my new iMac 2019 model