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Please make a screenshot or photo of the disk usage so we have a better picture at what is 'using' it mostly.
Please fill in your system specs
Please follow this tutorial
and download the tool. The tool will give you detailed information about your system specs, please fill in your system specs more completely.
In the left corner below in your post you find 'My System Specs'. After clicking it you can find a link a little below that says 'Update your System Spec', click on this link to get to the page where you can fill in your system specs.