I make a duplicate file of every image that I import, and I do this thanks to a Lightroom feature that takes every image I add to the catalog and automatically backs it up to an external hard drive. But once a month I go and make a real long-term archive of my image library; I put that month's images in cold storage for the indefinite future, removing them from my primary hard drive to keep it clean and fast, and storing them away in what I hope will be a permanent archive that lasts forever. I've written plenty about my overall backup plan—which involves storing duplicate files on hard drives, Blu-Ray disks and in the cloud—so, I won't beat a dead horse here. Suffice it to say that the external hard drive onto which I move my original files from my Lightroom catalog is the least archivally permanent storage option, but it's also the most convenient. And it's the drive that allows me to maintain relatively quick access to the files thanks to Lightroom.
I physically relocate my photos from one drive to another (by going into the regular old operating system file structure and dragging the folder from one drive to another) then I go back to Lightroom and look at those photos in the catalog. Now Lightroom says to me essentially, "Hey, where'd those files go?" It maintains the thumbnails for the images, but it displays the folder names grayed out and with an unmistakable question mark icon that indicates something is not right. That something is a missing folder.
But, you'll notice while the folder is missing, the previews remain. Those preview files, you see, live within the Lightroom catalog. So, even if you never reconnected your Lightroom catalog to those missing folders, you could still see the previews of those files. This is part of the genius of Lightroom's catalog structure; it keeps track of your photos even if you try to hide them from the program.
What Lightroom does not do is automatically find that missing folder of original image files. For that it needs your help. But it's easy enough to accomplish; simply right-click on the folder of missing photos within Lightroom and tell it to "Find Missing Photos." Then you simply navigate to where those photos now live. Point to the new folder, and Lightroom will then say, essentially, "Okay. That's on this other drive so I'm going to now keep track of your photos on that other drive."
What Lightroom really does is it adds that external hard drive to the list of drives indexed by the Library. I put my archived photos on that new drive in folders organized by date, and so Lightroom shows me those dated folders in list form in the Library panel, which means as long as those drives are connected I can still instantly access my image files through Lightroom, even though they're now living in their long-term external archive. If and when I take that drive online, I won't have instant access to the files through Lightroom (obviously) but the program will still point me to exactly where the files are stored, making it easier to know which drive I need to reconnect to access the originals.
As for lag time involved with this approach, it's not much of a factor. Because remember that the stuff I'm still working on remains in the master catalog on the main hard drive. Everything else is archived—old and put away—and when I click on that archived external drive to tell Lightroom to go and find the right folder of photos it does take a second to spin up the drive, but it's never much of a delay. And, it's way faster than the constant lagging I'd be facing if I kept my primary hard drive and Lightroom Catalog full of all of these image files. The system would simply get too cumbersome and slow me down on a constant basis. But since that tiny spin up delay is for the archive of old stuff, I'm not going in there on a regular basis. And, when I do need to find a photo from the archive, it's easy thanks to Lightroom.