Hi,
Your dream is reasonable. However, any time you use a “library” or “shoebox” style of app, especially on iOS, it’s going to copy the files into its container. So that’s why Photos does that (as well as Lightroom). It wants to make sure the files are always available and there are other technical reasons why it greatly simplifies the implementation (and therefore the reliability). Managing a single file here or there externally is not a problem – it’s the hierarchies and such that get a little crazy.
RAW Power currently copies the files as well. That is a temporary limitation that I plan to lift in a future release, where files can either be copied (which is essential for SD cards when they aren’t copied ahead of time by the user) and referenced in place (as you would like). I’d also like to support folders and such things, but that also creates a set of headaches that the current implementation side steps. In general, referenced file implementations are notoriously difficult to get right and the feature set that people want makes it a considerable investment in time.
For example, you mentioned wanting to move the files to another drive and have everything be visible. When files move to different drives (and different folder structures), apps then need features to smartly reconnect them. If you ever saw the reconnect interface in Aperture, it is a real bear. Lightroom on the desktop sort of throws up its hands at that case. Reconnecting also requires moving / renaming / synching the sidecar and database files which is hard to get right. None of the features by itself are intractable, it’s the combination that takes time (and the error cases – when moving files around, one has to handle file name collisions which mess up the connection to sidecars etc.)
There certainly may be apps already available that do what you want, though I’m not aware of them. If you want to use RAW Power for this, then I do plan to get there, sooner rather than later.