Every drive has its own Recycle Bin, and emptying one empties one

Delete a file from D: and it does not move to C:. Windows keeps a hidden $Recycle.Bin folder on every drive, with its own contents and its own size limit, and the Recycle Bin on the desktop is a view across all of them.

Why this catches people

• External drives have their own. Eject the drive before emptying and the files stay on it.
• Each drive has its own cap. By default a percentage of that drive, so a large drive holds a large amount of deleted data.
• Files larger than the cap skip it. Delete something bigger than the limit and Windows removes it immediately, with a different warning that people click through.
• Network drives never use it. Deleting from a mapped drive is permanent, immediately.

Seeing what is in there

foreach ($d in (Get-CimInstance Win32_LogicalDisk | Where-Object DriveType -eq 3).DeviceID) {
  $p = "$d\`$Recycle.Bin"
  if (Test-Path $p) {
    $s = (Get-ChildItem $p -Recurse -Force -File -ErrorAction SilentlyContinue |
          Measure-Object Length -Sum).Sum
    '{0,8:N0} MB  {1}' -f ($s/1MB), $p
  }
}

Run it in PowerShell. Each line is a drive holding deleted files. The folder is per user underneath, which is why another account's deleted files are invisible to you and still occupy the drive.

Emptying it properly

1. Connect every external drive you want cleared.
2. Right click the Recycle Bin on the desktop and choose Empty Recycle Bin. It covers every connected drive.
3. Or, per drive, open its $Recycle.Bin with hidden files shown and empty from there.
4. For the cap, right click the Recycle Bin, Properties, and set a size per drive.

When it still does not free space

If the Recycle Bin is empty and the drive still reports the same free space, the deleted files are being held by something else, and on Windows that is usually a shadow copy from System Restore, which keeps a reference to data as it was at the moment of the snapshot.