Can I transfer files to iCloud from an external hard disk?

If you're referring to iCloud Drive the answer is no. iCloud Drive is for syncing files between devices logged into the same Apple ID and keeps shadowed copies on any Mac that is logged in. That way they can access the files when off line.

Any file that is uploaded to the iCloud Drive gets copied to the local iCloud Drive folder on the Mac:

The iCloud Test File was uploaded from my EHD thru my browser and a copy was synced to my boot drive as you can see.

Actually you can, just copy them to the iCloud drive in finder, remembering that will also create copies of the files on the internal drive as well, also that changes to files on the external drive will not be reflected on the iCloud drive.

but I don't want to have all those files on my Mac.

The OP doesn't want them on his internal drive.

Old Toad wrote:

but I don't want to have all those files on my Mac.

The OP doesn't want them on his internal drive.

Then putting them on the iCloud drive will be of no use to him (iCloud drive keeps a mirror copy on your Mac). A different service is indicated.

An alterative would be to store them in Amazon's Web Services's S3, but:

(1) Although it's cheap to send files, it gets expensive to download them. So if the OP wants to do a lot of back and forth as opposed to simply archiving his files in the Cloud, AWS is probably not the answer.

(2) The AWS interface is daunting. Actually it's more than daunting. It's impenetrable, although there are third party solutions to help.

And alas, the OP has paid for a terabyte of iCloud.