@nileane Iris never modifies or moves your files. When you add a new source (you can have multiple sources!), the app generates thumbnails and indexes everything it can about your photos and videos.
Once that's done, you're free to eject the drive and go hide it in your attic or bank vault. You can then keep browsing and searching your library with Iris. Only need to re-attach the drive if you want the original files.
It's designed this way for folks (like me!) who don't have enough internal Mac storage for all their photos. It's also super helpful because when you do go looking for a specific photo, Iris can tell you which drive it's on.