Well... You can move those files the computer and profile you are currently using one drive on. Which it sounds like that is not what you want to do...
What I would do... This is not the most elegant... First make a new folder in one drive for your external drive "exltHDDbkup". Not sure If you have multiple computers or not. I would recreate a new profile just for this backup job. Make it a local account. Connect your hard drive. I am not sure if one drive will sync from the root of a drive... Just in case it does not... Create a folder and name it extHDDbkup... Log into the onedrive app with your windows account... Move the one drive folder from the c drive to the external drive... Sync just the extHDDbkup Directory... Move all files to that folder on the HDD and they will sync to the cloud... May not be the best instructions...
You can upload files through the browser as well...
Type d this on a phone... And I am in bed, but hopefully you can figure out what I am saying...
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