How to Find What Is Filling a Mystery 20 GB on Windows

Your drive shows 20 GB more used than your files add up to, and nothing in Downloads or Documents explains it. It is not hidden malware. Windows itself keeps several multi-gigabyte files that File Explorer does not show: the hibernation file, the page file, restore points, and a leftover copy of the previous Windows version. Storage settings and one search find nearly all of it.

Open Storage and wait for the scan

Open Settings, System, Storage. Wait for the bars to fill in, then click Show more categories if it appears. Each category is clickable.

Clean up Temporary files

Click Temporary files. Tick Windows Update Cleanup, Previous Windows installation(s) (this is Windows.old after an upgrade, often 10 to 30 GB), Delivery Optimization Files, and Recycle Bin, then click Remove files. Leave Downloads unticked unless you want that folder emptied.

Check System & reserved and shrink what you do not use

Click System & reserved. It lists the hibernation file, virtual memory (the page file), and System Restore. These are the usual mystery.

• Hibernation file (hiberfil.sys, up to about 40 percent of your RAM): only if you never use hibernate or fast startup, open Command Prompt as administrator and run:

powercfg /hibernate off

• System Restore: press Windows + R, run SystemPropertiesProtection, select the drive, click Configure, and lower Max Usage or click Delete to clear old restore points and their shadow copies.
• Page file: leave it alone unless the drive is nearly full. Windows sizes it for you.

Accept Cleanup recommendations

Back on the Storage page, click Cleanup recommendations. It groups large or unused files, unused apps, and files already synced to the cloud. Everything it lists is safe to remove.

Hunt for large files by hand

Open File Explorer, click This PC, and type in the search box:

size:>1GB

size:gigantic does the same for files over 4 GB. Click View, Details, then click the Size column header to sort. Usual finds: old virtual machine disks, video exports, installer ISOs, Outlook OST files, and OneDrive or iCloud caches under C:\Users\<your user name>\AppData. Shadow copies do not appear here; the System Protection dialog above covers them.

If Settings still cannot explain it. WinDirStat or TreeSize Free (both free) map every file on the drive, including hidden and system files, and show at a glance which folder holds the missing space.