How do I transfer data from old external hard drives to iCloud?

Your choices really are:

• Enable iCloud Drive on your computer and have the files on your Mac (you could optimise them to save space).
• Disable iCloud and load the files to iCloud through your browser, to avoid having the files on your computer.
• Use another cloud service for the files that does sync to your computer.
• Use another Apple ID, add it to a family share and load files through the browser. This would let you put the files into iCloud under another ID, but let you continue to use iCloud under your own ID without the files being on your computer.

Q: Why does 'dragging and dropping' directly from the G-Tech finder window (external drive) to the iCloud finder window icon in the sidebar seem to transfer everything to the local drive first? Is this Unavoidable?

The reason for this is because iCloud is intended to be more than space in the cloud. It's purpose is to be a hub so that all your devices can have exactly the same data on them. So your Mac will sync with iCloud which in turn will sync to your phone, you can then make changes on your phone which via the cloud will then get made on your Mac. You can 'optimise' some kinds of data to save space on your devices, optimised files are essentially just a place holder on your device, so that when you go to use that file, the full version will be downloaded from the cloud.

The reason I asked what you were trying to achieve might be covered by this iCloud feature.

There is another way you can use your iCloud drive just like a drive in the cloud, and that would be to turn of iCloud drive on your Mac and then load all the documents from your external drive to your iCloud drive through iCloud.com using your browser. You'd need to do this a few files at a time though.

Can I upload to the cloud through both portals simultaneously?

Yes, but I'm not sure it will achieve what you want. If you enable iCloud drive on your Mac, any files you upload through the browser will appear on your computer drive.

Both routes lead to the same place.

Sorry, I'm not with you there. Anything that's in iCloud Drive “should” be on your Mac also.

Also, if you disable iCloud Drive, the only way to add files would be through the browser.

It would help to know exactly what you are trying to achieve here.

This is the option for enabling/disabling iCloud Drive.

If you turned it off, any files within iCloud Drive would be archived to a folder on your Mac.

Do you use iCloud Drive for other documents on your computer?

(Our conversation is a little out of sync, I missed your second to last response as I was typing my last response.)

Which approach?