how to prepare for transfer to new iMac 24"

What I Should do, is only backup the documents and files you really want to keep, I would copy them to an external drive, and set up the new iMac anew, so without putting over a backup and put the documents you want to keep manually over. This presents transferring Things you don’t want to have or that are giving problems

Below are the instructions you need to migrate the old Mac's data to a new Mac. If your external HD is not working, please replace it. Links to suggested EHD's are in the information below:

When you buy a new Mac the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes  and follow the on-screen prompts and it's done! It is THAT simple.

If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts. 

The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

A Mercury Elite Pro would require an additional adapter for use with a new M1 iMac.

Choosing a SanDisk Professional 2TB G-Drive ArmorATD™ - Apple would be a good option. It includes a USB-C cable for your new iMac, and an older-style USB-C to USB-A cable to use with your old iMac.

You can follow WheelieNick's advice to only transfer your files, or you could backup your old Mac with Time Machine, then restore that backup to the new Mac.

Jack

Normally you need 2-3 times the amount you have stored on your Mac. For most people a 4TB model is plenty of capacity and offer a lot of future growth. Here is the direct link (1.) and also I've included a link to Amazon (2) to order the adapter I mentioned you will need.

1. https://eshop.macsales.com/item/OWC/ME3QH7T4.0/
2. https://www.amazon.com/Syntech-Adapter-Thunderbolt-Compatible-MacBook/dp/B07CVX3516/ref=sr_1_1_sspa?crid=1PA31GD1OULAQ&keywords=usb+adapter+to+usb+c&qid=1688239469&sprefix=usb+adapter%2Caps%2C233&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

By the way, I do not receive any compensation if you buy either product. I recommend them because I know from experience that they work.

Your welcome. I'm a tired old man (age 69) so I get it.