Moving large quantity of files from external HDD to iCloud

No. iCloud Drive is primarily a synchronization service, not offline storage. As space becomes low on your Mac, files will offload to iCloud only. But, the files will still persist on your Mac by design.

You do know that the iCloud Drive is a syncing feature and not an off computer storage facility, right?. Any file you put in the iCloud Drive is mirrored in your Home/Library/Mobil Documents folder.

So if you want to get files off your MBP to free up space use an external SSD like OWC's ENVOY PRO ELEKTRON or its ENVOY PRO FX.

As others have noted, no; you need to space available on your Mac's boot drive to be able to then upload to iCloud. Also, as a comment, why are you wanting to use iCloud in this fashion? If to archive the 1TB of files, iCloud isn't an archiving service; others like BackBlaze and related are for that.

No, as removing them from iCloud on the Mac (to make space for the new files) will remove them (within 30 days, I believe) from within iCloud. As separately noted, iCloud isn't an archiving service, whereas a dedicated service would allow for what you suggest to happen more readily.

You wrote, "removing them from iCloud on the Mac (to make space for the new files) will remove them (within 30 days) from iCloud."

I'm still confused about how this works. Why would he need to remove them from *iCloud* on the Mac? To make space for new files, he would only want to remove the files locally. But you're saying that if he deletes the data from his Mac's SSD, they can't stay in iCloud Drive either?

Why couldn't Apple design iCloud Drive to sync and also simply store data? Why are they making me use Dropbox as well?

Thanks for answering my question. I am not entirely happy with how iCloud Drive works. It would be nice if Apple let you upload the data to iCloud Drive and keep it there as long as you kept an alias of the file on your device. Hopefully they'll do that one day. Perhaps you or someone else can chime in on why that would not be feasible.

Apollo471 wrote:

Why couldn't Apple design iCloud Drive to sync and also simply store data? Why are they making me use Dropbox as well?

Tell Apple what missing features you'd like restored or new features added to the system via Feedback - macOS - Apple.  

It might be slow and tedious, but couldn't he or she transfer individual files from the external HDD to the Mac, then upload then offload the file to iCloud Drive?

Correct. Files deleted locally are also deleted in iCloud. iCloud is primarily a synchronization service. Apple didn't design any place to have an offline file store