Migration Assistant

Yes, precisely. Connect the External Hard Disk to the iMac (use a USB cable) which may require an adapter depending on if the iMac is a 24" or 27" iMac. If it's a 24" you will need a USB adapter (available on Amazon for about $10) to connect. Also, the External Hard Disk is likely formatted so you can read with the iMac but not write to it.

Finally, it is wise to buy a high quality External Hard Disk such as a OWC Mercury Elite Pro and use that as your backup External Hard Disk using Time Machine. If you are not familiar with Time Machine please read Back up your Mac with Time Machine to learn more about it.

Have a look at > https://support.apple.com/en-us/HT204087

Also, do not use Wi-Fi to ever run MA it stalls almost every time. Using an Ethernet cable is okay however moving your important data to an external HD and then copy and paste to the Mac is much easier. You can learn some tricks too, for example if you put all of. your photographs in one folder and then drag that folder to the Photos app on the iMac all the photos will be imported automatically into the Photos app and then you can begin to organize them.