Quick answer: If C is full but you cannot see large files, hidden AppData, system files, restore points, virtual disks, and sync folders are common causes.
Why this happens
File Explorer does not always make storage obvious. Hidden folders and system-managed data can be large while normal folders look small.
Safe steps
- Enable hidden items only for inspection, not deletion.
- Check Storage settings by category.
- Review AppData, OneDrive, virtual machine files, and restore point usage.
- Use a disk space analyzer for folder sizes.
- Clean with Windows tools before manual deletion.
What to avoid
Do not delete hidden folders just because they are hidden or large.
Use DiskTidy for review-first cleanup
Download DiskTidy to scan common cleanup locations and large files locally before deciding what to remove.