Difficult to think what might be the issue. On my system there is no disk activity if Calibre is idle. Ideas that occur to me are:
- Calibre is running a background task. This might happen if Calibre has to recreate the metadata.opf file (used to recover the Calibre metadata database if it gets corrupted) stored with each book. However this task normally runs at a low priority so that it does not impact performance.
- Windows is swapping Calibre to/from disk. You do not mention what might be running in parallel with Calibre. It seems that you should have enough RAM to stop this happening, but looking in Windows Task Manager should help determine this.