Hydrus creates an 'imageboard' on your local drive by importing the images you want (either selections from your hard drive or direct download). When it imports the images that you already have saved to your hard drive, it creates a copy of each image into a set of subfolders within the Hydrus program folder. It's up to you if you want to delete your old collections once you've imported them into Hydrus. These files are named using a hash (original filename discarded) and the tags are stored in a database so you must thereafter use Hydrus to manage tags for those files.

As far as I could tell, Hydrus does not add metadata to your already existing files including any tags recognized by Windows Explorer. It will create tags using file and/or folder names if you enable those options before import, so if you already sort media using folders and subfolders, Hydrus can use your already existing names as tags.

The author encourages us to abandon the use of file and folder names for cataloging large numbers of media files and I tend to agree, especially when you generate content faster than you can tag it (as in photography or if you enjoy downloading zillions of files at a time).

However, for many years I have already been using my file system to manage several archives of different media and desire a catalog manager that tags files in place, preserving my filenames and folder structure and which works on top of (or along side of) my file system. So I therefore use Hydrus for the bulk of my assorted download content and yet I continue to seek a suitable tagging utility for my already well-maintained collections as I'm not ready to let go of what's both trusted and familiar on my system.

NOTE: I only spent an hour or two evaluating this program on Windows 7 Pro with a small subset of my image archive, which really isn't much time to speak authoritatively about it, I may have misunderstood or missed something in the process so therefore you should evaluate it yourself. Overall, it was a bit fun even though I didn't immediately understand everything I was looking at.