It sounds like a 40Gb Drive, it is quite normal not to actually 'see' all 40Gb some of which is used by the System.
Hover the mouse cursor over the drive's icon in My Computer then tell us what you see - it should show Amount Used and Amount Free.
Dell XPS 13 9370 (2018) review
No need to scan sketches into your computer with Moleskine's new smart pen
Streaming : Netflix ou Amazon Prime Video ?