Windows 10 version 1903 will automatically reserve around 7 GB of device’s disk space to store updates and optional files to prevent Windows Update failed due to insufficient storage, starting from May 2019 Update. You can recover Reserved Storage with following steps.
Check whether Reserved Storage enable
Step 1: Go to Settings > System > Storage.
Step 2: Click Show More Categories.
Step 3: Click System & Reserved.
Step 4: You will see Reserved Storage section with 7+ GB of storage space in use if Reserved Storage is enabled.
How to Disable Reserved Storage
Step 1: Go to Start and typing regedit to open Registry Editor.
Step 2: Navigate to the following key:
Step 3: Double-click
Step 4: Change Value Data from 1 to 0 then click OK.
Step 5: Close Registry Editor and reboot Windows 10 to apply the changes. You may have to wait few weeks before Windows installs an update and deletes the reserved storage.