One or more things may be happening here:
- Your drive may be partitioned into two or more virtual drives. This is unlikely if no other drive letters are showing up on your PC besides the external.
- You may have multiple backups stored away on the C drive which are hidden.
- OneDrive may not be configured to store in the cloud and optimize local storage. However, if you only have 25GB, this is also unlikely.
- You may have old email (with attachments like photos, etc.) stored away which needs to be purged or off-loaded.
- Other ...
I suggest you install Disk Analyzer Pro or similar from the Microsoft Store. It is free