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Sorry to say you have a serious problem brewing. When a HD gets too full the computer will stop functioning and then you have a real problem. iCloud is a service which mirrors what is on your Mac. It is designed for file sharing, not for storage or backup. You need to quickly get yourself an external disk which will be connected via a wired USB cable normally provided by the disk vendor. Then you can copy files onto it before deleting them from your HD to create needed available storage space. For simple storage you don't need the more expensive SSD. A lower cost mechanical drive will suffice. It may need to be reformatted before use. If so, use only Disk Utility to do so, not any maintenance app that comes with the disk. They always cause problems for Macs.