"Store in iCloud ... overwrite Documents question.: I wish to free up space on my spare 2015 MacBook Pro. One option is to store Desktop and Documents to my 1 TB iCloud Drive but I am worried these will overwrite my current Desktop and Documents in iCloud Drive. Or will there be another 'folder' created in iCloud Drive? I dare not take the risk."
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Troubleshooting Overwriting of Files:
A. Nothing will be overwritten. What is shown is on iCloud or downloaded to your computer. So, all will be shown as one device. Have no worries.
B. Download All of iCloud:
One option would be to download all of iCloud, and then cash all that is in there. Once trashed, all would safely sync.
Use my User Tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - Apple Support