Icloud - Sync folder from a external harddrive

Hello TheMikeSweden,

Thanks for reaching out to Apple Support Communities. We've reviewed your question and it looks like you want to make iCloud Photos use your external drive and sync from there. It's understandable that you'd want this resolved to keep all your photos safe and up to date, so allow us to assist with that. Please start by taking a look here: Designate a System Photo Library in Photos.

If you have follow-up questions, please let us know. We're here for you. Have a fantastic day!

Thanks for getting back with us, TheMikeSweden.

If you are wanting to manage your files and documents - and not your iCloud Photos - perhaps you would like to use iCloud Drive? This tool lets you securely access all of your documents from your iPhone, iPad, iPod touch, Mac, and Windows PC. This article explains more: Access and manage your files in iCloud Drive.

Let us know if you have further questions. We're here to guide and assist.

As a test, have you tried making an alias of a file that is on the external drive and then moved the file alias to the computer's Documents folder to see if that synced properly?

Have you tried creating an alias of the Documents folder that is on the external hard drive and moved the alias to the Documents folder on your computer?