How can I display ALL the files with a certain tag even if the files are located in the different folders?
I've tried the file collections method, but it would appear that they aren't dynamic, since if I add a new file that matches the search terms of a specific file collection, that new file won't be visible in mentioned file collection, and I have to keep re-making file collections to match the new contents of my hard drive.
Tools > Find Files > Advanced is the way.
You can make a button which runs a Find with a predefined filter so you don't have to set it up manually every time and it will act a bit like going to a folder, but it will take a while each time as your drives will need to be scanned to find the files with the tags.
Opus does not maintain an in-memory, dynamic, real-time, always-updated database of "files that exist anywhere with tag X".
Windows Search, along with its indexing service, can be used for some metadata on some types of files (e.g. DOC, MP3, JPG) to speed up finding them.
That's what I tried but it didn't search any drive apart from the one I was currently in, and I added a "full path" operator with a wildcard (*). So how do I make it look in all drives?
Are we talking about when using the GUI or when using the Find command to automate things?
Tools > Find Files > Advanced GUI
You can add multiple folders to the Find In list at the top of the panel (that's why it's so big).
If you click the button above the list that shows a folder browser (or double-click Add Folder To Search in the list itself), there's also an option in there to search all local harddrives.