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This is not typical or intended behavior. It sounds like your qBittorrent configuration or a system-level caching mechanism is causing temporary files or cache to be written to the C: drive despite your torrents being seeded from the D: drive. Here are some steps to troubleshoot and possibly resolve the issue:

1. Verify Cache Configuration in qBittorrent

Go to Tools > Options > Advanced:
Ensure Disk cache is properly configured. If you disabled it entirely, try setting it to a fixed value (e.g., 512MB or 1GB) to see if it behaves differently.
Check if OS cache is enabled or disabled. You may need to experiment with toggling these settings, even though you've already tried them.

2. Check the Temp File Directory

qBittorrent might be using the C: drive as a temporary file storage location.
In Tools > Options > Downloads, verify the Temporary folder setting. If it's on the C: drive, change it to a directory on the D: drive.
Ensure "Keep incomplete torrents in a separate folder" is unchecked or points to the D: drive.

3. Investigate Windows Page File Usage

If your system is using the page file extensively during seeding, the C: drive might be written to due to virtual memory.
Check the page file settings:
Go to System Properties > Advanced > Performance > Settings > Advanced > Virtual memory.
Set the page file size manually or move it to the D: drive.

4. Analyze the "Phantom" Space Usage

Since TreeSize doesn't find the files, it's possible that the writes are to hidden or system-protected directories, or temporary files.
Use tools like Resource Monitor (Disk tab) or Process Monitor (Procmon) to track qBittorrent's disk activity. Focus on writes to the C: drive and identify the files/directories being written to.
Check the Windows Temp folder (C:\Windows\Temp) for large files or excessive activity.

5. Check for System-level Cache Behavior

Windows Disk Write Caching or a third-party tool might be interfering.
Open Device Manager > Disk Drives > Properties > Policies and adjust the Write Cache Settings for both drives.
Test whether disabling or enabling "Turn off Windows write-cache buffer flushing" affects the behavior.

6. Revert to an Older Stable Version of qBittorrent

If the issue started with version 5.0.1 and persists with 4.6.7, consider reverting further back to an earlier stable release (e.g., 4.4.x) and testing.
Sometimes newer versions introduce bugs or regressions, especially with changes to disk I/O handling.

7. Perform Disk Health Checks

Ensure there are no underlying issues with your drives:
Run chkdsk /f on both drives.
Use tools like CrystalDiskInfo to check the health of your SSD and HDD.

8. Check System Logs

Open Event Viewer and look for disk-related warnings or errors under Windows Logs > System.
Look for messages about write failures, cache issues, or low disk space warnings related to the C: drive.

9. Test with Another Torrent Client

If nothing resolves the issue, test with another torrent client (e.g., Deluge or Transmission) to confirm whether the problem is specific to qBittorrent or a broader system issue.

By systematically working through these steps, you should be able to identify whether the problem is rooted in qBittorrent, Windows caching behavior, or another system-level interference. Let me know if you'd like guidance on any specific step!