Why is my C drive suddenly full but I have nothing on it?

Why is my storage so full all of a sudden or my disk suddenly at 100%?? Sometimes after upgrading or installing major Windows 11 or Windows 10 updates, the C drive may suddenly run out of space without reason. This usually happens because Windows stores old system files, update caches, and backup data in case you want to roll back. These files can easily take up tens of gigabytes and remain hidden from normal view. Luckily, you can safely clean them up and free space without affecting your system. How to fix my C drive suddenly full? Here comes two simple ways to solve the problem.

1. Delete Old Windows Update Backups

Go to Settings > System > Storage > Temporary files.

Select Windows Update Cleanup and remove it.

This can free up 10–30 GB.

2. Remove Windows.old Folder (Rollback Files)

Open File Explorer and check if C:\Windows.old exists.

If you don't plan to roll back, delete it via Disk Cleanup (it won't delete by normal right-click).

After these steps, your C drive should recover significant space while keeping Windows 11 running normally.